Onyx is Andy's horse and constant companion.
Onyx is a large brown horse with a white blaze marking on her nose. The horse is a constant companion to Andy, and acts as his main means of transportation. Onyx has a lavish stall in the main living quarters of Andy's ship, and seems to be just as comfortable riding inside human buildings as she is outside.
Onyx is shown to have above average intelligence for a horse. She seems 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. Andy also claims that the horse can play a decent game of chess.
Andy renames Onyx "Jiroumaru" after adopting his new samurai persona.