stable hunter

A unicorn that actively seeks an established, committed couple to date or have sex with but in an unethical way. Stable hunters will demand the couple date them, and only them, forbidding any other sexual or romantic connections.

Also may be called stable chasers

Jack: Tom and Betty are polyamorous and have been married 5 years. Lucy asked them out for a date tonight

Jill: I’ve heard Lucy is a stable hunter. They should be careful.