I am very sorry if this has been posted before but I only see a lot of things about different things about this.
Most of you probably know this but for the ones who don't, first get your costume fromt he inn keeper or a fellow friend/person in wow willing to use his hallowend wand on you. Once you get your costume all you have to do if you don't like it is join BG and then leave. Each time you enter or come out it will change. But be sure not to die orit will go as all you BG people know. If you are a form that can't mount go in BG and/or out and you will change form hopefully into a mounting type straight away. I havn't done much testing on this but not sure if you are still unable to mount when you come out of BG as a whisp or bat as I ahvnt tried this yet. But when I done it I was a booty bay goblin unable to mount. I entered BG and I was a leaper gnome and then when I cameout I was a Ninja. Remember not to die.
Just thought I would add this even though most of you probably know. May aswell add something that not everyone may know aswell. even if its one person that benafits from this^^.
Have fun.
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M n it's s HT!!!