Just as some help for new people: The best places for this are Warcraft Mounts, Warcraft Pets, Wowhead and of course Petopia.
Note: The tag retired is used for pets and mounts that used to be available in-game but aren't anymore even if they still are available elsewhere (such as the old racial mounts that may still creep up on e-Bay for horrendous sums). Pets and mounts from old TCG sets, collector's editions, etc. are not marked as retired even though they are hard/nigh impossible to get because they were never available in-game to begin with.
Note #2: I've split the Hunter Pet - Mini Pet Combo list in two parts: Multi-Color and Single-Color Pets. The reason is that there are some Battle Pets that appear in various colors in the wild. I don't want to clutter up the list by naming those pets several times wherever they fit. So they get their own list. Their colors are stable after catching them, they aren't color-changers.
Hunter Pet – Mini Pet – Mount Combo
Bears:Any white bear – Baby Blizzard Bear (retired), Poley – Big Blizzard Bear, White Polar Bear
Any black bear – Dun Morogh Cub – Big Battle Bear, Black War Bear, Amani War Bear (retired), Amani Battle Bear
Any brown bear - Hyjal Bear Cub - Armored Brown Bear
Any dark brown bear - Darkshore Cub - Darkmoon Dancing Bear
Boars:
Any silver Draenor boar– Lifelike Mechanical Frostboar – Giant Coldsnout, Armored Frostboar
Cats:
Duskfang – Panther Cub – Night saber (Alliance only/retired), Swift Zulian Panther
Frostsaber Pride Watcher – Winterspring Cub – Winterspring Frostsaber (Alliance only)
Shadowclaw – Nightsaber Cub – Striped Nightsaber (Alliance only)
Shango – Savage Cub – Swift Zulian Tiger (retired)
Cranes:
Any blue crane – Azure Crane Chick – Azure Riding Crane
Core Hounds:
Any red core hound – Core Hound Pup – Core Hound
Direhorns:
Any blue direhorn – Pygmy Direhorn – Spawn of Horridon
Dragonhawks:
Sunblade Dragonhawk – Blue Dragonhawk Hatchling – Blue Dragonhawk (Alliance only)
Feral Dragonhawk – Golden Dragonhawk Hatchling – Red Dragonhawk (Horde only)
Goats:
Any white goat – Summit Kid – White Riding Goat
Nether Rays:
Any purple nether ray – Nether Ray Fry – Purple Riding Nether Ray
Any red nether ray – Fledgeling Nether Ray – Red Riding Nether Ray
Raptors:
Any purple raptor – Deviate Hatchling – Violet Raptor (Horde only)
Any red raptor – Lashtail Hatchling, Razzashi Hatchling – Armored Razzashi Raptor
Any black raptor – Obsidian Hatchling – Venomhide Ravasaur (Horde only)
Any green raptor, King Krush – Ravasaur Hatchling – Emerald Raptor (Horde only)
Any blue raptor – Gundrak Hatchling – Darkspear Raptor (Horde only)
Dart (or any orange raptor) – Darting Hatchling – Swift Orange Raptor (Horde only)
Ravagers:
Any red Draenor ravager – Bloodthorn Hatchling – Grinning Reaver
Riverbeasts:
Any pale riverbeast – Albino River Calf – Trained Riverwallow
Any blue riverbeast – Periwinkle Calf – Sapphire Reiverbeast
Any green riverbeast – Flat-Tooth Calf – Mosshide Riverwallow
Silithids:
Any green silithid – Silithid Hatchling – Green Quraji Battle Tank (only in AQ), Blue Quiraji Battle Tank (only in AQ), Ultramarine Quiraji Battle Tank
Tallstriders:
Mazzranache, Deviate Plainstrider - Mulgore Hatchling, Lovebird Hatchling - Swift Lovebird
Any purple Tallstrider – Darkmoon Hatchling – Swift Forest Strider
Turtles:
Any green turtle – Speedy, Emerald Turtle, Softshell Snapling – Riding Turtle
Any green dragon turtle – Spiny Terrapin – Black Dragon Turtle
Any brown dragon turtle – Wanderer's Festival Hatchling – Blue Dragon Turtle
Wolves:
Deathmaw, Grovepaw – Worg Pup (colour changer brown/black) – Brown Wolf (Horde only)
Any black worg – Worg Pup (colour changer brown/black) - Black Wolf (Horde only)
Ripp – Tito – Timber Wolf (Horde only)
Any silver Draenor wolf – Swift Frostwolf (Horde only), Armored Frostwolf – Frostwolf Pup
Felbound Wolf – Fel Pup – Infernal Direwolf
Mini Pet – Mount Combo
Birds of Prey:Ji-Kun Hatchling – Clutch of Ji-Kun
Firewing – Solar Spirehawk
Dread Hatchling – Armored Dread Raven
Blazing Firehawk – Pureblood Fire Hawk
Brooms:
Feline Familiar - Flying Broom (requires Hallow's End)
Cats:
Spectral Tiger Cub – Spectral Tiger, Swift Spectral Tiger
Guardian Cub - Winged Guardian
Dragonkin:
Emerald Whelpling – Emerald Drake
Bronze Whelpling – Bronze Drake
Dark Whelpling – Black Drake
Tiny Crimson Whelpling – Red Drake
Netherwelp – Azure Netherwing Drake
Azure Whelpling – Blue Drake
Frosty – Icebound Frostbrood Vanquisher
Proto-Drake Whelp – Red Proto-Drake
Emerald Proto-Whelp – Green Proto-Drake
Nexus Whelpling – Azure Drake
Onyxian Whelpling – Onyxian Drake
Soul of the Aspects - Heart of the Aspects
Lil' Tarecgosa - Tarecgosa's Visage
Thundering Serpent Hatchling – Thundering Jade Cloud Serpent
Wild Golden Hatchling – Golden Cloud Serpent
Wild Crimson Hatchling – Crimson Cloud Serpent
Wild Jade Hatchling – Jade Cloud Serpent
Elekks:
Pint-sized Pink Pachyderm – Purple Elekk (Alliance only)
Peanut – Grey Elekk (Alliance only)
Meadowstomper Calf – Great Blue Elekk (Alliance only), Shadowhide Pearltusk
Gronnlings:
Dwarf Gronnling – Gorestrider Gronnling
Runtish Gronnling – Sunhide Gronnling
Gryphons:
Gryphon Hatchling, Wildhammer Gryphon Hatchling – Golden Gryphon/Swift Blue Gryphon (Alliance only)
Hawkstriders:
Eggbert – Purple Hawkstrider/Swift Purple Hawkstrider (Horde only)
Hippogryphs:
Hippogryph Hatchling, Cenarion Hatchling – Cenarion War Hippogryph
Horses:
Argent Squire + Argent Pony Bridle - White Stallion (Alliance only/retired), Swift White Steed (Alliance only), Quel'dorei Steed (Alliance only)
Mechanostriders:
Mechanopeep - Red Mechanostrider (Alliance only)
Phoenix:
Phoenix Hatchling – Ashes of Al’ar
Dark Phoenix Hatchling – Dark Phoenix
Rams:
Little Black Ram – Black Ram (Alliance only/retired)
Raptors:
Fossilized Hatchling – Fossilized Raptor
Reindeer:
Winter Reindeer – Fresh Holly/Preserved Holly (temporary)
Seahorse:
Sea Pony - Abyssal Seahorse
Wind Riders:
Wind Rider Cub – Tawny Wind Rider/Swift Red Wind Rider (Horde only)
Hunter Pet – Mini Pet Combo
Multi-Color PetsBeetles:
Beetle & Dung Beetle can fit any black, orange, red or brown beetle
Crystal Beetle can fit any black, orange, red or white beetle
Gold Beetle can fit any orange or brown beetle
Twilight Beetle can fir any black or pink beetle
Scarab Hatchling can fit Temple Scarab and any purple-red, green-red or light blue scarab
Stinkbug can fit any black, orange, pink or brown beetle
Crabs:
Shore Crab & Strand Crab can fit any bronze, white, blue or red crab
Moths:
Forest Moth & Oasis Moth can fit any blue, green, white or yellow moth
Fungal Moth can fit any blue, grey or white moth
Silky Moth can fit any blue, grey, green, white or yellow moth; also comes in a unique beige & blue color
Swamp Moth & Tainted Moth can fit any blue, white or yellow moth
Parrots:
Parrot can fit Albino Eyegouger/Cackle, Dusky Lory and any red parrot
Scorpids:
Crunchy Scorpion can fit Duneclaw Broodlord or any brown scorpid
Leopard Scorpid can fit any black, brown or white scorpid
Scorpid can fit any black, brown, red or yellow scorpid
Stripe-Tailed Scorpid can fit Duneclaw Broodlord or any black, brown or white scorpid
Spiders:
Dusk Spiderling can fit any grey, olive or jungle spider
Spider can fit any olive or jungle spider
Desert Spider can fit any brown or red bone spider
Warp Stalkers:
Lizard Hatchling & Horny Toad can fit Gezzarak, Warp Stalker or any white warp stalker
Horned Lizard can fit Daggertail Lizard or any white warp stalker
Spiny Lizard can fit Chasm Stalker, Daggertail Lizard and any green or white warp stalker
Single-Color Pets
Axebeaks:
Any black axebeak – Axebeak Hatchling
Any green axebeak – Junglebeak
Any red axebeak - Bloodbeak
Jungle Axebeak – Fruit Hunter
Basilisks:
Any red outland basilisk – Scalded Basilisk Hatchling
Bats:
Any brown bat – Tirisfal Batling, Bat
Any red bat – Vampiric Batling (retired)
Bears:
Any black diseased bear – Infested Bear Cub
Beetles:
Any red beetle – Lava Beetle, Fire Beetle
Any blue beetle – Savory Beetle
Temple Scarab – Sand Scarab
Any light blue scarab – Tol'vir Scarab
Carrion Birds:
Any blue condor – Blue Mini Jouster
Any brown condor – Gold Mini Jouster
Any white condor – Dragonbone Hatchling
Any black vulture – Fledgling Buzzard
Upland raven – Gilnean Raven
Young Red Teroclaw – Teroclaw Hatchling
Cats:
Any dark striped cat – Black Tabby Cat
Any black cat – Bombay Cat
Any orange striped cat – Cornish Rex, Orange Tabby Cat
King Bangalash, Hawkbane – Silver Tabby Cat
Young Frostsaber – White Kitten
Any brown cat - Darkmoon Cub
Jade Guardian – Jade Tiger
Skarr – Onyx Panther
Dreadsaber – Cheetah Cub
Clefthooves:
Banthar – Leatherhide Runt
Chimaeras:
The Razza, Ronak – Albino Chimaeraling
Core Hounds:
Chromaggus, Chromehound – Chrominius
Crabs:
Any red crab – Strand Crawler, Lava Crab, Spirebound Crab
Any blue crab – Spirit Crab, Emperor Crab
Any white crab – Shore Crawler
Any silver crab – Moonshell Crab
Ancient Spineclaw – Spineclaw Crab, Zangar Crawler
Any blue spiked crab – Frostshell Pincher
Any red spiked crab – Ironclaw Scuttler
Any green spiked crab – Kelp Scuttler
Any orange spiked crab – Fen Crab
Ghostcrawler – Ghostshell Crab
Cranes:
Any green crane – Jade Crane
Crocolisks:
Any grey old-world crocolisk, Gosh-Haldir – Chuck
Any green old-world crocolisk – Snarly
Any brown cataclysm crocolisk – Muckbreath
Any yellow old-world crocolisk – Toothy
Spirit of Koosu (retired) – Viscidus Globule
Direhorns:
Any green Direhorn – Stunted Direhorn
Dragonhawks:
Any red dragonhawk – Red Dragonhawk Hatchling
Any violet dragonhawk – Phoenix Hawk Hatchling
Eagles:
Any brown eagle – Tickbird Hatchling
Any white eagle – White Tickbird Hatchling
Any bald eagle – Imperial Eagle Chick
Any undead eagle - Blighthawk
Foxes:
Any red fox – Fox Kid
Any white Fox – Arctic Fox, Arctic Fox Kit, Alpine Fox
Gorillas:
Any dark grey gorilla – Bananas, Baby Ape
Hydras:
Mystlash Hydra – Gahz’rooki
Any blue thunder hydra – Hydraling
Chimaeron – Lanticore Spawnling
Kaliri:
Any brown kaliri – Kaliri Hatchling
Any black kaliri – Ikky
Any red kaliri – Veilwatcher Hatchling
Monkeys:
Gib the Banana-Hoarder - Darkmoon Monkey
Moths:
Any blue moth – Blue Moth
Any green Moth – Red Moth, Crimson Moth, Garden Moth
Aspatha – White Moth, Ashwing Moth
Any yellow moth – Yellow Moth, Gilded Moth, Amber Moth
Any grey moth – Grey Moth, Luyu Moth
Frenzied Manafeeder – Royal Moth
Any pink moths – Dusty Sporewing
Any red & white moth – Crimsonwing Moth
Any blue & white moth – Cerulean Moth
Owls:
Any brown owl – Great Horned Owl
Any black owl – Hawk Owl
Skettis Kaliri, Avian Warhawk - Brilliant Kaliri
Any grey owl – Crested Owl
Hell-Hoot – Snowy Owl
Any green owl – Jade Owl
Ban’thalos – Sentinel’s Companion
Parrots:
Ol’Beaky – Cockatiel
Monstrous Parrot – Green Wing Macaw
Aotona – Hyacinth Macaw
Brilliant Macaw – Miniwing
Sunwing Squawker – Senegal
Any green and yellow parrot – Polly
Porcupines:
Lowland Porcupine – Silent Hedgehog
Docile Porcupine – Malayan Quillrat, Malayan Quillrat Pup
Bristlespine – Porcupette
Smokey Porcupine – Clouded Hedgehog
Quilen:
Bronze Quilen – Lucky Quilen Cub
Ravagers:
Any green ravager – Ravager Hatchling
Any black Draenor ravager – Deathwatch Hatchling
Any blue Draenor ravager – Icespine Hatchling
Verdant Ravager – Mossbite Skitterer
Any buff Draenor ravager – Thicket Skitterer
Raptors:
Takk the Leaper (or any grey raptor) – Leaping Hatchling
Any yellow raptor – Razormaw Hatchling
Rhinos:
Any grey rhino – Stunted Shardhorn
Riverbeasts:
Any red riverbeast – Mudback Calf
Scorpids:
Any bronze scorpid – Durotar Scorpion
Duneclaw Broodlord - Scorpling
Any grey scorpid – Mechanical Scorpid
Seagulls:
Any seagull – Rustberg Seagull, Sea Gull, Sandy Petrel
Serpents:
Shimmerscale, Needlespine Shimmerback – Cobra Hatchling
Shale Spiders:
Jadefang – Tiny Shale Spider, Emerald Shale Hatchling
Any purple shale spider – Amethyst Shale Hatchling
Any red shale spider – Crimson Shale Hatchling
Deep Spider – Topaz Shale Hatchling
Spiders:
Any crystal spider – Smolderweb Hatchling, Crystal Spider
Any jungle spider – Forest Spiderling, Feverbite Hatchling
Any black widow – Widow Spiderling
Any purple spider – Amethyst Spiderling, Twilight Spider
Any red spider – Jumping Spider
Any olive spider – Skittering Cavern Crawler
Any green tarantula – Venomspitter Hatchling
Any light grey bonespider – Giant Bonespider
Any orange fire spider – Molten Hatchling, Blazing Cindercrawler
Spirit Beats:
Spectral Porcupette – Gumi
Harmonious Porcupette – Hutia
Vengeful Porcupette - Degu
Sporebats:
Any green sporebat – Tiny Sporebat
Any purple Draenor sporebat – Shadow Sporebat
Moonglow Sporebat – Glowing Sporebat
Stags:
Any reindeer – Winter Reindeer
Any striped tan gazelle – Gazelle Fawn
Felrot Courser – Infected Fawn
Turtles:
Any red turtle - Darkmoon Turtle
Warp Stalkers:
Any white warp stalker – Ash Lizard, Warpstalker Hatchling
Daggertail Lizard – Plains Monitor, Spiky Lizard
Chasm Stalker – Twilight Iguana
Wasps:
Any yellow firefly – Firefly, Shrine Fly
Any purple firefly – Tiny Flamefly
Any green firefly – Darkmoon Glowfly
Blacksting – Effervescent Glowfly
Frenzied Swamplighter – Violet Firefly
Swamplighter Drone, Kliaa – Sapphire Firefly
Swamplighter Worker – Waterfly
Swamplighter Queen – Swamplighter Firefly
Any blue hook wasp – Twilight Wasp
Any buff hook wasp – Bone Wasp
Any bright green hook wasp – Wood Wasp
Any red hook wasp – Bloodsting Wasp
Any yellow hook wasp – Amberbarb Wasp
Water Striders:
Stream Strider – Aqua Strider
River Strider – Mirror Strider, Dancing Water Skimmer, Eternal Strider
Fen Strider – Legs
The Black Stalker, Coilfang Strider – Coilfang Stalker
Wolves:
Any black saber worg – Fjord Worg Pup
Any white wolf – Moon Moon
Gara – Lost Netherpup
Worms:
Any green silkworm – Jungle Grub
Any red silkworm – Imperial Silkworm
Bombyx – Gu’chi Swarmling
Hunter Pet – Mount Combo
Bats:Echo Hunter – Armored Bloodwing
Boars:
Any black Draenor boar – Blacksteel Battleboar
Any grey-brown Draenor boar – Great Greytusk
Young Rocktusk – Domesticated Razorback
Trained Coldsnout – Trained Rocktusk, Frostplains Battle Boar
Bristling Hellboar – any red Draenor Boar
Deathtusk Felboar – Diresnout Felboar
Cats:
Sambas – Golden King (Alliance only)
Duskstalker – Spotted Frostsaber (Alliance only)
Frostsaber – Striped Frostsaber (Alliance only)
Shango – Swift Zulian Tiger (retired)
Frostsaber Huntress – Ancient Frostsaber (Alliance only/retired), Swift Frostsaber (Alliance only)
Green Pandaren Tiger – Red Shado-Pan Riding Tiger
Red Pandaren Tiger – Green Shado-Pan Riding Tiger
Savage – Blue Shado-Pan Riding Tiger
Clefthooves:
Any blue Draenor clefthoof – Trained Icehoof
Any red Draenor clefthoof, Grom’kar Warbeast – Bloodhoof Bull
Cranes:
Any white crane – Golden Riding Crane
Any black crane – Regal Riding Crane
Direhorns:
Any red direhorn – Crimson Primal Direhorn (Horde only)
Any brown direhorn – Golden Primal Direhorn (Alliance only)
Any blue direhorn – Spawn of Horridon
Dragonhawks:
Any yellow dragonhawk - Amani Dragonhawk
Goats:
Any black goat – Black Riding Goat
Any brown goat – Brown Riding Goat
Nether Rays:
Any green nether ray – Green Riding Nether Ray
Marshfang Ripper – Blue Riding Nether Ray
Aether Ray – Silver Riding Nether Ray
Quilen:
Green Portent – Imperial Quilen
Raptors:
Drakkari Raptor Mount – Mottled Red Raptor (Horde only/retired)
Any black devilsaur – Black Primal Raptor
Any red devilsaur – Red Primal Raptor
King Krush – Green Primal Raptor
White Devilsaur – Bone-White Primal Raptor
Any green Draenor raptor – Vicious War Raptor (Horde only)
Ravagers:
Any red Draenor ravager – Grinning Reaver
Rhinos:
Any white rhino – Wooly White Rhino
Rylaks:
Any yellow rylak – Iron Skyreaver
Scorpids:
Brown Madexx/Venomscale Spitter - Kor'Kron Annihilator (Horde only)
Any orange armored scorpion – Amber Scorpion
Silithids:
Any yellow silithid - Yellow Quiraji Battle Tank (only in AQ)
Talbuks:
Bach’lor – Cobalt Riding Talbuk, Cobalt War Talbuk
Talbuk Sire – Dark Riding Talbuk, Dark War Talbuk
Talbuk Thorngrazer – Tan Riding Talbuk, Tan War Talbuk
Any lavender Talbuk – Silver Riding Talbuk, Silver War Talbuk
Talbuk Doe – White Riding Talbuk, White War Talbuk
Any black Draenor talbuk – Shadowmane Charger
Any tan Draenor talbuk – Swift Breezestrider
Any purple Draenor talbuk – Trained Silverpelt
Any red Draenor talbuk – Breezestrider Stallion (Horde only)
Any silver Draenor talbuk – Pale Thorngaze (Alliance only)
Tallstriders:
Any turquoise tallstrider - Swift Shorestrider
Any white tallstrider - Swift Springstrider
Turtles:
Any blue turtle – Sea Turtle
Any gold dragon turtle – Brown Dragon Turtle
Elder Sea Dragon – Great Brown Dragon Turtle
Any camo dragon turtle – Green Dragon Turtle
Rankbite Ancient – Great Green Dragon Turtle
Chillwater Turtle – Purple Dragon Turtle
Brineshell Snapper – Red Dragon Turtle
Bloodtooth – Great Red Dragon Turtle
Krosh – Great Black Dragon Turtle
Elder Shoreshell Snapper – Great Blue Dragon Turtle
Great Turtle Furyshell – Great Purple Dragon Turtle
Water Striders:
Lake Strider – Azure Water Strider
Glimmer – Crimson Water Strider
Wolfs:
Old Cliff Jumper – Dire Wolf (Horde only)
Shattered Hand Warhound – Black War Wolf (Horde only)
Any white worg – Arctic Wolf (Horde only/retired), Moonfang (requires Darkmoon Faire)
Any red worg – Red Wolf (Horde only/retired)
Any grey Draenor wolf – Smoky Direwolf, Ironside Warwolf (Horde only)
Any black Draenor wolf – Garn Nighthowl
Any brown Draenor wolf, Riding Wolf – Trained Snarler
So this is it. I hope I didn't miss anything. ^^ Have fun collecting your combos and feel free to post some pics here.
Edited because I missed something.