Hmmm. Trying to think what the best option from the supernatural section is...you might just be a unique being... You could make it work as like a...Reincarnated if one of the forms was unhinged and bill it as a beast within kind of character a-la jeckle and hyde. 

Or. 

An unliving whose soul was crafted as both genders or neither, as a result it is capable of rearranging its appearance at will and putting one of two personalities.