In my mind, you hire the oxen as part of the carts, therefore you are hiring the oxen. No, not individually, but where did I state "I only hire one oxen, and nothing else with it"?
By the way, I agree that horses should not be paraphernalia, I'm just playing Devil's Advocate to make sure you guys use actual arguments that are based on logic, not just "It'll make the game more girly".