A cart is built 'on' a steed to extend its cargo and passenger capacity. You must mount the steed to start the project. You cannot lock it in any way. It shows up on the objects page like an engine in a vehicle. There is an option to disassemble it. Carts can be added to the larger steeds including the domesticated horse, zebra, camel, or kiang.
See also: saddlebag, small cart