As a sociologist who specializes in human sexuality I am qualified to say there is nothing wrong with fapping to ponys. Human sexuality is a complex thing, it manifests in very...interesting ways (Over 1200 confirmed fetishes for example). Masturbation itself isn't bad either, it has no health or lasting mental side effects(other then the various factors involved in orgasm). If you do it alot you can develop a zinc and nickle deficiency but thats only in extrem cases. In fact, animal related fetishes have been traced back over a very long period of time and are far more common the one would think(not the most mind you, that would actually be object fetishism and light BDSM).

Oh, and to add...this is far...FAR from the strangest fetish i've encountered (Theres a fetish for playing dead, for being cripple, having a full bladder, for cars, for near drowning, for quicksand)