Here are all the colours of gryphons, hippogryphs, owl cats and wyverns and their current availability status, i.e. whether they exist in the game and can be attacked and therefore potentially tamed. We still have no idea which will be tameable, other than gryphons, which are a definite :
Gryphons
Available (just one colour) :
Gryphon_Skin : Wild Gryphon etc.
Unavailable:
Gryphon_SkinBlack : same as black mount but no harness, does not seem to be used in game
Gryphon_SkinWhite : same as white mount but no harness, does not seem to be used in game
SpectralGryphon : The unused wild model version of the spectral gryphon mount
EliteGryphon : The wild model version of the mount, seems to be unused so far
Hippogryphs
Available (5 colours, over two models) :
Hippogryph2Aszuna [sic] : used in the Broken Isles
Hippogryph2Valshara : used in the Broken Isles
HippogryphBrownSkin : Spotted Hippogryph etc.
HippogryphSkin : Thunderhead Hippogryph
HippogryphWhiteSkin : Antilos (Rare) and Antilus the Soarer (Rare) - one of these two rares could be switched to one of the unavailable skins
Unavailable:
Note that a couple of these may have appeared in the 7.1 database and so may appear on tameable beasts by the time the new family is released.
EliteHippogryph : this is the skin used for the hippogryph mount from the Blizzard Shop. This skin has not yet been added to the Hippogryph2 model folder, but it's identical in format to the skins used for that model and would give the new hippogryph a third colour if it was copied across.
Hippogryph_Arcane : used on a new mount, but a wild version of the model exists, it's just not been found in the wild yet
HippogryphBlueSkin : Frayfeather Stagwing exists but is friendly to Alliance. Frayfeather Patriarchs have been removed, Empyrean is unavailable as the quest was removed, but he could be made a rare somewhere.
HippogryphPurpleSkin : Frayfeather Hippogryph exists but is friendly to Alliance. Frayfeather Skystormers seem to have been removed
HippogryphFelSkin : This skin was used for a TCG mount but is unsaddled and makes a perfect wild beast. Fitting for Legion!
BurntGryphSkinRed (on non-saddled model) : would make a great rare in Firelands areas or southern Hyjal
PyrogryphSkin (on non-armoured model) : would make a great rare in Firelands areas or southern Hyjal
Owl Cats
All Available :
OwlCat_Black
OwlCat_Blue
OwlCat_Brown
OwlCat_Orange
And in case Wyverns / Windriders are made tameable...
Wyverns / Windriders
Available:
WyvernSkin : Pridewing Wyvern etc.
WyvernSkinBlue : Sorrow Wing (Rare)
Unavailable:
WyvernSkinGreen : used only by Heartrazor, a neutral quest mob, but he's not attackable. If he needs to be left that way, then perhaps the green skin could be given to the Pridewing Patriarch, a rare that seems to be have been removed in Cataclysm but could be returned
SpectralWyvern : The non-mount version of the spectral wyvern mount. Seems to be unused.
EliteWyvern : non-mount model version of the spectral mount. Seems to be unused