The mickey mouse platy has a marking near its caudal fin that resembles Mickey Mouse's head, or three black spots adjacent to each other. Here is a picture:
http://www.liveaquaria.com/product/p...21&pCatId=1068
I have heard of an occasional platy being aggressive, and from what I have read of others' experiences, nothing can really be done about it (adding, removing fish, changing tank environment, etc.) I have had some success lowering the temperature a few degrees, as with some fish (not all, check their individual requirements) a higher-end temperature can contribute to aggression.