You decided to study the stallion, so you walked to it's stall, it was munching on some hay from a hay pile, it's tail periodically flicking about, your main focus was on it's ass, you reached out and felt it, it was like soft velvet, the stallion let out a soft grunt and turned it's head, looking at you, then he goes back to eating, not seeming to mind someone by his hindquarters.
You shrug and smile as you softly stroke the anus at hand: it was big, bigger than a normal stallion's that's for sure, you suddenly heard a call from the farmer. "I'm going out for a few hours, got some things to do, hay bales to buy for the horses y'know, I'll be back in about 4 hours, ya'll stay as long as ya like, take your time." Then you heard a truck start and the farmer left.
It was then, right after the farmer left, that the stallion picked his head up and looked back at you, then he nuzzled a bit feeling you stroke his ass, you smiled from the nuzzle and petted his head softly, the stallion whinnies soft and motions at its big anus, you draw your attention back to it, and its tail raises, you can see his anal hole clear as day, it had to be twice as big as normal! You blush a bit, then...