Darkmoon Faire is comming in not too long now, and I found this interesting bit on Wowhead.http://www.wowhead.com/npc=71992
Moonfang is summoned by killing Moonfang Snarlers. When enough have been killed, Moonfang Dreadhowls will spawn, and after slaughtering enough, Moonfang will appear to tackle your raid.
Sounds to me that Moonfang will not be remotely as rare as the Darkmoon Rabbit, since players can actively work to spawn it.
Edit: From what Im hearing, the Moonfang mount is supposedly -ONLY- useable during Darkmoon Faire, further hinting to her being a far more common boss.