Snarler is a level 39 rare black wolf that can be found patrolling north of the road in eastern Feralas. I have indicated where he can be found in red on the map below.
He drops the typical green item and a decent amount of experience for people leveling.
This particular wolf used to be highly sought after by hunters because he originally had an incredibly high amount of base magic resistance. That was fixed by Blizzard many moons ago however. He currently shares the same abilities as all of the other wolves in game but still makes a pretty nice looking pet. If your looking for this particular model and color and are having trouble finding him; there are level 16-17 elite wolves in Redridge Mountains, and level 32-33 wolves in the Hinterlands that look exactly like him.