Well, as promised, my additions:
The Burning Crusade
Teeny Lurker Above
Pet Type: Aquatic
Location: Drop: The Lurker Below
Description: "A small kraken. Doesn't it just look cute, with its huge teeth and cries of death?
Tiny cries of death!"
Wrath of the Lich King:
Ulduar:
XR-200J-G5
Pet Type: Mechanical
Location: Drop: Flame Leviathan
Description: "A prototype miniature version of the infamous Flame Leviathan siege tank."
Razorscale Hatchling
Pet Type: Dragonkin
Location: Drop: Razorscale
Description: "Before the augmentation of armor plates on Veranus, Ignis tested the procedure out on one of her hatchlings. The procedure was too intense for the hatchling to cope with, and the whelpling ended up incapable of separating friend from foe."
Steelbouncer
Pet Type: Mechanical
Location: Drop: The Assembly of Iron
Description: "This miniature version of Steelbreaker and his titanic iron brethren was first created when tampering with various prototypes. The madness consuming Ulduar has caused a form of amnesia within this creature; it no longer knows what it has to attack."
Tiny Sanctum Kitten
Pet Type: Magic
Location: Drop: Auriaya
Description: "These teeny tiny feline-shaped bundles of metal joy forgot whom to attack after being separated from their mistress, Auriaya."
Miniature Command Unit
Pet Type: Mechanical
Location: Drop: Mimiron (Cache of Innovation)
Description: "Not yet programmed, these useful miniature versions of Mimiron's 'Aerial Command Unit' have overwhelming firepower."
Private Xazev
Pet Type: Magic
Location: Drop: General Vezax
Description: "This creature is bluntly said, a wondrous anomaly. An inexplicably tiny, harmless version of the imposing General Vezax of the Faceless that will indubitably intrigue many researchers and scientists."
Organic Saronite
Pet Type: Magic
Location: Drop: Yogg-Saron
Description: "A harmless replica of Yogg-Saron; something no-one but the Old God itself may ever be able to explain. They do say imitation is the sincerest form of flattery, but it's doubtful that that phrase holds any water if the one you want to flatter is yourself."
Alternatively, because no matter how cool a tiny Yogg-Saron would be, it seems unlikely, sadly:
Faceless Hatchling
Pet Type: Magic
Location: Drop: Yogg-Saron
Description: "A footsoldier deemed unworthy of Yogg-Saron's command due to its tiny size and harmless semantics; its tie to the Death God was forcibly severed, leaving it to wander aimlessly."
Tiny Hologram
Pet Type: Magic
Location: Drop: Algalon
Description: "A tiny hologram that looks like Algalon the Observer, left by him as a gift to the brave adventurers that bested him and showed him that there may yet be a chance for Azeroth to cleanse the corruption on their own."
Living Constellation
Pet Type: Magic
Location: Drop: Algalon
Description: ".. No words."
Cataclysm:
Dragon Soul:
Lesschok
Pet Type: Elemental
Location: Drop: Morchok
Description: "A tiny version of Morchok, too immature or too powerless yet to assume a proper own will, it followed Morchok, assuming the gigantic elemental would take care of him."
Tentacle of Go'Rath
Pet Type: Magic
Location: Drop: Warlord Zon'ozz
Description: "One of Go'Rath's tentacles, previously at the Faceless Warlord's beck and call now follows you. Perhaps it has acknowledged you?"
Sma'jijh the Sleeper
Pet Type: Magic
Location: Drop: Yor'Sahj the Unsleeping
Description: "Unlike its master, this little faceless mindbender does sleep whenever it desires."
Tiny Amalgamation
Pet Type: Magic
Location: Drop: Spine of Deathwing
Description: "A miniature amalgamation of Deathwing's corrupted blood. It seems disoriented.."
Elementium Fragment
Pet Type: Magic
Location: Drop: Madness of Deathwing
Description: "A mixture of the Old God's tentacles, Deathwing's vile blood, and small pieces of broken elementium armor plating."
Pandaria
Technically not a new pet, but rather a model change:
Jumping Spider
Pet Type:[/b] Beast
Location: Pet Battle: Jade Forest
Description: "This terrifying and highly venomous species leaps onto its prey from the Jade Forest's dense canopy."
Ability added: Leap/pounce, because.. - jumping spider.
I felt the current Jumping Spider abilities and model didn't do it justice. Obviously, the Nerubianspiderling model is not completely correct either, but with its legs in a position that seem ideal for leaping/pouncing, and huge eyes, they may as well just say that they are the WoW equivalent of Jumping Spiders.
As for them looking Nerubian.. well, we have mouse pets with rat models and lizard/monitor pets with warpstalker models. I think the new model would be an upgrade either way. It fits in well with Jade Forest's surroundings, and it looks great!
Besides, quite a lot of people want that model to be available in some shape or form. I still want it as a tameable spider (despite possessing 6 legs), but a battle pet is a good start!