It's Crusty the Crab! Crusty (who is clearly a Spongebob Squarpants reference) is a level 32 non-aggressive crab that spawns off the coast of northern Desolace. I have indicated his patrol area in red on the map below.
He drops the typical green item and a decent amount of experience for people leveling.
He is also one of only two rare crabs currently in game. The only other rare crab is Ghostcrawler, the level 85 spirit beast crab in Vashj'ir. If you are a hunter and are simply interested in taming a crab of this particular color, there are several others to choose from in game. They range from level 2 to 80 and can be found in Durotar, Wetlands, Tanaris, Howling Fjord and Kelp'thar Forest in Vashj'ir. This is also the same model and color that goblin hunters start out with.
Crusty can be found with NPCScan. You will need to add it, his npcid is 18241.
You can also use this macro to help find and tame him:
/cast Tame Beast
I'm not sure what his spawn timer looks like, but my guess is that its not too long. If it's like most rares on Azeroth since the Cataclysm, it is probably somewhere between 30 min - 2 hrs.