Imagine:
Draft horse, really big, real tall, huge dick (like 3 feet). Youve got a girl, not quite sure shes willing, but shes tied to this draft horses underbelly, dick between her and the horse. Shes tied on snug but she can slightly move back and forth. Her muzzle is shoved into the urethra, then her head tied to keep it that way. The shaft runs between her breasts, and her nethers are squished against his balls. A few days like that and she finay gets used to it, drinking cum, smelling like his musk all over, etc.