Someone who prefers the act of sex more with an animal than another human being. Many are dirty, sick minded people and tend to leech onto society. The usual method of a Beastanist is to rub peanut butter on their genitalia and let an animal(usually a dog) to remove said peanut butter, resulting in a forced attempt of oral sex.
"Hey John, did you have hot sex with that chick you went out with last night?"
"No Chad, Im a Beastanist. I have no sexual desire for woman. Geez man, get that shit right."