The black gemstone model you mention is very cool. It's actually used on a hostile NPC as well as that friendly pet you saw: Swarming Guardians that spawn during the fight with Auriaya in Ulduar. Unfortunately last time they were tested on the beta (quite a few patches ago) they were untameable and had no aggro table.
There's actually a whole series of gemstone tigers of different colours. Unfortunately they don't exactly look natural, so I guess it's hard to fit them in. I suggested they could put some in Deepholm as kind of elemental beasts, but it was probably too late for that suggestion even if they noticed it.
If you're interested in seeing the whole list of gemstone models, I just copied this from the untameable skins list:
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TigerSkinAmber: glossy brown, purple eyes
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TigerSkinAmethyst: glossy purple, green eyes
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TigerSkinBlackGem: glossy black, red eyes - untameable
Swarming Guardian
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TigerSkinDiamond: glossy grey-white , blue eyes
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TigerSkinJade: glossy green, red eyes
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TigerSkinRuby: glossy red, blue eyes
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TigerSkinSaphire [sic]: glossy blue, yellow eyes
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TigerSkinTurquoise: glossy aqua, yellow eyes