Some people may want to know, haven't seen anything about it here yet.
Straight from petopia forums...
http://forums.wow-petopia.com/viewtopic.php?f=3&t=7036
http://forums.wow-petopia.com/viewtopic.php?f=3&t=7058
In short;
Mobs that retain a unique visual appearance in death can be tricked into keeping it when tamed. The trick is simply to tame them at the exact moment they die, and then cast "resurrect pet" on them to make their corpse vanish. After this is done, you can safely resurrect the pet and it will retain it's skin, as seen in these two pets (oily hawk, ghost carrion bird). It could be worth watching for pets with unique looks that don't lose them when they die (most do, unfortunately).
The ghost bird vanishes on its own and so doesn't need to be murdered, the bug lies in the fact that if a mob vanishes as you tame it, the game doesn't erase its unique skin. This person was creative enough to "force" the oily hawk to vanish.