Brawl'gar Arena is the Horde's Brawler's Guild. Creatures and fighters from all over Azeroth and Outland have been gathered to participate in bloody arena deathmatches with heroes of the Horde.
The building housing the arena is also known as the Ring of Valor, notable for being the site of a duel between Thrall and Garrosh Hellscream during the Scourge Invasion.
The full combatant list is on the Brawler's Guild article.