G'nathus is a massive underwater eel found in Townlong Steppes. He circles around Shan'ze Dao along with many of his Spawn of G'nathus friends.
G'nathus also drops a battle pet, Spawn of G'nathus.
Fighting G'nathus is similar to fighting the Whale Shark in Vashj'ir, though G'nathus is harder to kite. It is recommended to take 10 or so people at least, with a few healers and a tank, but it can be done with much less - even soloed if you are extremely bored.