There are at least three car rental companies on property - 4 if you count Alamo/National as two.
Alamo/National has locations at the Car Care Center (near the MK) and at the Swolphin. Avis has a location in the Hilton Hotel on Hotel Plaza Blvd.
Dollar has a location at one of the other hotels on Hotel Plaza Blvd, can't recall which one off hand.