A colour-changing pet is one which picks a differently-coloured skin each time it's summoned.
This was actually a bug which was fixed in 5.0.4. Pets tamed before the patch maintain this trait. Those same mobs tamed after the patch do not change colour.
I'm pretty sure I have the complete list here. In some cases there were a few differently-named mobs which used the exact same skin.
- Dredge Worm
- Enthralled Crustacean
- Marsh Crocolisk
- Borer Beetle
- Jeweled Scarab
- Sand-Husk Scarab
- Pyreshell Scuttler
- Sentry Worg
- Rabid Hyena
- Baradin Fox
- Venomtip Needler
Curious Jungle Monkey - There were a few which used the exact skins/colours, but this one vocalizes when clicked.
- I have all of them.
Yes, I am the proud owner of every colour-changing pet in the game.
(Also every non-beastmaster ghost pet, but that's for another post.)