Flesh Golems are constructs literally made of flesh. These can include a specific type of flesh golem made or experimented on by members of Dalaran, as well as abominations of the Scourge.
However, unlike other Golems they do not share spell immunity. And they also have random quotes, a n angered grunt and also saying "We done waiting". They also have Shockwave and Cripple abilities, in addition to their massive health pool, making them the most powerful unit in the game.