Groom John takes prissy pony Sherlock to the country so he can be properly bred by a suitable stallion.

He ends up as the main attraction at a play party, tied to the breeding stand in a stable yard, arse-up and helpless as the biggest and most well-endowed pony-boys John can find try to prove their worth to the assembled onlookers by filling John's pretty little filly with as much cum as possible.

Humiliation play is a bonus.

(No non-con, but role-play dub-con is fine)