Onyx was Andy's horse and constant companion.
Onyx was a mainly brown horse with a black mane, a white blaze marking on her nose. Onyx had a lavish stall in the main living quarters of Andy's ship, and seemed to be just as comfortable riding inside human buildings as she was outside.
Onyx had above average intelligence for a horse. She seemed to be able to operate and ride in elevators without any help from humans, riding to the top of City Hall to pick up Andy after his fight with Spike Spiegel. Andy also claimed that the horse could play a decent game of chess.
Andy renamed Onyx "Jiroumaru" after adopting his new samurai persona.