A horse. Sometimes used for a mule or donkey. Originally a burro.
An irreverent derivative of Oldsmobile, referring to an equine animal that may eat oats and is ridden by humans.
Originated in "Boys Life" magazine (the official magazine of the Boy Scouts of America) when boys sometimes irreverently address a fictional character, Pedro the burro, as "Oatsmobile". Pedro is the "mail burro" for that magazine and answers boys' letters.
Are you going to ride that oatsmobile?