Grom the Paunch
Grom, AKA Paunch of the Misty Mountain, AKA "Da Fat", was among the greatest Greenskins to have ever lived in the Warhammer Fantasy setting.
The Legend
Once upon a time, there was a group of Goblins. No more interesting than any others of their race, they found themselves without a WAAAGH to join and thus decided to have an eating contest. Deciding to up the ante, one of their number dared another, a small Goblin named Grom, to ingest troll meat. Raw troll meat, which regrows faster than almost any being can digest it. Theoretically, this should have killed him since only beings like Ogres or other trolls can accomplish such a feat and not burst. After several weeks (during which the competition went on) of excruciating pain and constant sharting, Grom's belly bloated as the meat regrew inside him. Goblins gathered around him, waiting for the inevitable explosion of gore. It never came. Grom's body adapted, and he somehow came to digest the hunk of meat at the exact rate it regrew, granting him regenerative abilities and making him larger than any Goblin, and even most Orcs which almost immediately made him the Boss of his small clan, the Broken Axe. Ten years later, he had become Warboss of almost all Greenskins in the southern Badlands which he added to his growing WAAAGH (because of course, you gotta have a WAAAGH). After claiming leadership of a large chunk of the Night Goblins by literally crushing their Warboss beneath his paunch, he lead an attack on the Dwarf lands and wiped out multiple Holds before having his minions chip away a statue of the Dwarf god Grungi into his own likeness. The Dwarfs regrouped and with reinforcements attacked WAAAGH Grom, fighting a three day battle to an ultimate stalemate. The Dwarfs sent messages to the Empire for aid, while the Goblins simply waited for more of their race (and their larger kin) to come from afar to see the spectacle of the fucking massive Goblin who's da biggest (resulting in most Night Goblins at the time joining him).
The Emperor at the time, Dieter IV, responded by immediately removing the capital of the Empire from Nuln, and moving it to the other side of the Empire to Altdorf where it remains to this day (End Times notwithstanding). The entry in the Book of Grudges as a result ALSO remains to this day. The High King of the Dwarfs responded by commanding the Dwarfs seal themselves within their Holds, and after a short time of rampaging through the Dwarf lands and unable to find anything to fight, the WAAAGH moved on.
Grom had an archenemy in the form of Git Guzzler in earlier editions, although it was unclear if Grom ever actually knew Git existed or what happened to Git after Grom died.