Karoma is one of the most sought after pets in WoW right now. It was introduced with 4.0, is a Spirit Beast and appears to have a 6 to 12 hour spawn timer. He spawns in one of 6 locations randomly, and from what I've read he (along with all rare spawns) remain in his location until ether tamed or killed. He drops only the grey Tear of Loyalty, indicating quite clearly that he is meant by Blizzard to be a hunter pet only.
He is not technically a unique skin. He matches the skin of the
Ancient Grimtotem Spirit Guide, a previously tamable pet that was made tamable, and then un-tamable during BC. Blizzard obviously never intended for them to be tamable, as they only existed for 7 seconds, too short for a normal Tame Beast cast. However, a few hunters found a way to tame them, and Blizzard allowed them to keep their hard earned pets.