A pet on the head (human or otherwise) that starts by placing one's hand on the forehead and petting downward over the nose, as if one was petting a horse, but using excessive speed, force and/or pressure. It can be used as a greeting, or as a way to troll one's friends in person.
Teenager #1: And then she said, I have canc---
Teenager #2: HORSE PET! *violently horse pets teenager #1*
Teenager #1: What the f#@$ was that?!