In the Mopatop's Shop episode "Horns" Billy the Bull sent a puzzle to Mopatop, "How can a bull play a tune on a bicycle", if Mopatop are able to answer the puzzle with something from the shop Billy would reward Mopatop with a prize.
When Billy arrived to the shop, he liked Mopatop's answer so much that he played a tune while riding on a bike around the shop.
In "Bath Buddy," Billy came to Mopatop's Shop to look for dirt when he mom sent him to get a bathtub as Billy hates taking baths. This proves to be a problem when he meets Boris the Bath Buddy who manages to convince him into joining him in a bath.
His design is similar to a bull that was seen in The Muppet Show, but with different eyes, hair, and horns.