Someone who loves their pets more than human beings or, at the extreme, someone willing to kill a human to save a lower animal's life.
Billy Ray blew-up the cosmetic test lab, killing several employees, in order to discourage the company from testing products on lab rats and hairless beagles. He was a total petaphile.
1.
Person: I cant believe youre a petaphile
Petaphile: IDK what you are talking about, dogs are fucking hot!
Person: What the fuck...
2.
Person 1: Did you hear about the PETA member who was found to have trapped children in animal cages for his own sick pleasure?
Person 2: Yeah. I know about that.
Person 1: What a sicko...
Person 2: Dont call me a sicko just cause I have sex with children.
Person 1: I wonder why the PETA didnt do anything about their members problem... wait, WHAT!?