Barnaby is Mister Evil’s pet crab. He’s a rather laid back kinda guy. When Mister Evil found him, Barnaby was trying to catch cheese from a river. Mister Evil pointed out that it is impossible to catch cheese from a river, and Barnaby realized that he made a good point.
He was so impressed by Mister Evil’s smarts (and the bit of cheese he gave him) that Barnaby decided to follow him. Every now and then, Mister Evil will make some outrageous demand of Barnaby, like “Kill all the beggars in Orgrimmar for me, Barnaby” or “Kill all the gold farmers in Dead Wind Pass for me, Barnaby,” but Barnaby is pleased to oblige, for it often means that he’ll be able to feast on his favorite dish—elf.