Hogger is a minor villain in World Of Warcraft and one of the first elite-bosses an Alliance player can face: for this reason he has become somewhat of a legendary villain in the community due to the fact many low-level players have fought him and died (being inexperienced with dealing with such mobs): it is jokingly claimed that Hogger is one of Warcraft's most powerful villains of all time (in reality he is one of the weakest).
Storywise Hogger is simply an abnormally large and powerful gnoll, a type of hyena/human hybrid and has been terrorising the Alliance sufficiently for Stormwind to put a bounty on his head - allowing brave groups of adventurers to go forth and hunt the monster down in order to strike a blow not only to him but his band of piratical followers that are causing problems as well.
Following the events of Cataclysm, Hogger has been upgraded to a dungeon-boss in Stormwind Stockade, making him more infamous than he already was. Adventurers brave enough to end the riot Hogger began will eventually face the Gnoll King himself. In the end of the battle, Hogger is slain once and for all.
Hogger also appeared (briefly) as a raid-boss in a Blizzard Con where he was grotesquely overpowered to the point of "god mode" - this was played for comedy value and indeed Hogger proved to be unbeatable. Thankfully such an insanely powerful Hogger will not be appearing in the real game, being created as a bit of fun for the event.
During BlizzCon 2011, two guilds from the Alliance and Horde raced to see who could defeat every boss in the Firelands on heroic difficulty first. The Alliance guild lost the race, and as they prepared another attempt on Ragnaros Blizzard spawned Hogger in the middle of their group, who proceeded to wipe them all out.