A few days ago I came across a Wood Wasp with an unusual color skin. Normally they are light green with a brown underside. But this one was a dark black-green with a purple underside and dark green wings. It was one of the back rank additional pets that joined the battle. My team was low on health and I knew I couldn't catch it with them so low. I forfeited and tried again after healing up. The color was normal again. I thought I'd lost my chance, but I kept forfeiting, and lo and behold, the dark black-green one appeared again in the second rank after quite a few forfeits. I managed to catch it that time! The color stayed even after catching it and summoning it out.
I've never seen a front line wood wasp that color before, and I haven't seen a back line wood wasp that color again. But I know it's possible because I caught one. The skin is the dark green/black hook wasp skin that is used by the Stingtail Worker wasps in Spires of Arak. This skin is not used by any of the other wasp battle pets that I'm aware of.
It looks like the Dark Green Hook Wasp on the Petopia website for WoW Hunter pets. (I can't post a link yet because I just made an account on this site. You can google Petopia.)
I've also seen two skins for the Icespine Hatchling - the pale skin seems to be rarer than the darker skin from my personal experience. I've also seen the Parched Lizard come in two sizes (as opposed to two colors). I've also seen a few (critter) Frostfur Rats that were black instead of white, but none of them were catchable battlepets. But if they can spawn in that skin color, can they be caught in that skin color too?
Has anyone else seen differences in size or color in the new WoD battle pets? Are there any old pets that have 'rare' skin colors or sizes as well?