What determines your character's form in WoW?

Ok I'm super new to this game, my older brother wanted me to try it. So this is probably a super noobie question but here goes... I finally made it to the high twenties (level-wise) and I was doing a couple quests in Darkshire, where there are tons of werewolf-looking creatures. On this one quest, I had to kill tons of them and after awhile I realized my night elf rogue had turned into a werewolf himself!!! He started running around on four legs and he was bigger! Is that normal? Definitely wasn't part of the quest... Sadly, he turned back into his usual night elf form after a little bit. Anyways, so I was just wondering why that happened and how I can do it again with other creatures. Do you just have to kill boat loads of them? I notice tons of other players with forms I've never seen before, but I figure it has to do with their character type....