A rideable is any animal, object, or vehicle that a player can equip and use as a mode of transportation. They don't, however, do anything to amplify or improve the overall speed of the avatar's movement, and serve more a cosmetic purpose.
Currently, there are only two stores available on the YoWorld map that sell rideables. The Car Store specializes more in vehicles or special transportation arrangements. The Yo Valley Ranch specializes more in animal and creature-based transportation. Both places sell rideables for both YoCoins and YoCash.