The "Darkmoon Faire," a classic week-long event in World of Warcraft, started yesterday with daily quests to do, exotic vendors to visit, mini-games to play and a level 103 world boss to kill. That might be tough with the current level cap of 90, but it's not impossible. All you need is a lot of people.
Moonfang being a level 103 boss is not a bug, just a weird coincidence. Warlords of Draenor comes out next Thursday, with the level cap raised from 90 to 100, but as usual, Blizzard already updated a ton of things in patch 6.0.2 and it looks like the Darkmoon Faire was part of that update as well.
Large-scale world events are the best in WoW (or in any MMO) so definitely check this out while you can, take nonsense screenshots like the one below (I love how you can tell it's a long fight because someone's selling repairs there) and try not to crash the server.