The story
Coined in a 1995 episode of the animated TV show The Simpsons named "Lisa's Wedding", in which a Renaissance fair sideshow falsely promotes a normal rabbit as "a horse with the head of a rabbit and the body of a rabbit". Possibly inspired by fanciful names for creatures from Medieval Latin (e.g. basilisk, cockatrice).
Etymology from Wiktionary, CC BY-SA 4.0.