Patch 2.3 hit and Dustwallow Marsh got a make-over, Hunters were delighted to learn that the Ghost Wolves in that area were tamable. This was not Blizzard's intention, but they saw nothing wrong with letting it happen.
Many Hunter's flocked to the Marsh to get one of the coolest looking pets in the game. And now we learn, they look even cooler in combat. And hotter. And greener.
Mania's Arcania reports that the Ghost Wolf pet turns different colors while under different combat effects. In addition to the ones in the picture, the pet can also turn purple while under a Curse.
My guess is this is an unintentional side effect of the animal not having a solid texture skin over the 3-D model frame. All pets
If Blizzard is really on the ball, they will create other semi-transparent mobs, pets and maybe a temporary shapeshifted form for players that takes advantage of this. They already do it with the Spectral Tiger Mount and the Shadow Priest form, why not on other models that would be affected by colorizing of buffs/debuffs? How fun would that be?
EDIT: Added in examples of current in-game semi-transparent models.