Because fiction can be anything you (or anyone else) wants regardless of what it is originally and that is the beauty of it. No one is right or wrong for how they depict a fictional character as they are not real and thus cannot be subject to bigotry or violence. Fiction is a great way to express yourself or your interests in as it is safe to do so regardless of what it is.
Anyway, point being, any depiction of Misty ais valid as she's a cartoon horse.