Flesh giants, as the name states, are giants made out of flesh. It would appear to be the next step up from the more common abominations. Most flesh giants are simply giants pieced together from flesh, however, some have been outfitted with mechanical parts. The prototype flesh giant Grobbulus, the first of the flesh giants, is encountered in Naxxramas and others can be encountered in Northrend. They are also thought as the Scourge's counterparts to the fel reavers used by the Burning Legion.
The same dark sorcery used to command the Scourge's powerful abominations is utilized to even greater effect in the harsh and distant realm of Northrend. Here the Lich King has focused his malefic powers on flesh giants: nightmarish creatures that make abominations look tame by comparison. Scourge minions assemble flesh giants and abominations in a similar manner, piecing them together out of body parts from numerous corpses. Unlike abominations, though, the "spare parts" used to create flesh giants are harvested from multiple types of fearsome Northrend giants.
These living battering rams employ tremendous brute strength in carrying out the inexorable will of their loathsome master. Those who oppose the flesh giants take their lives into their own hands... and risk a fate even worse than death.