You rub the damp, stained tighty-whities against your penis. The wet, sticky fabric rubbing against your organ feels unreal, hardening so fast in pleasure, it almost feels like your cock is growing. But as you feel the cum welling up inside your shaft, slowly pushing out at the cusp of orgasm, you realize it is. You moan loudly in pleasure as you continue to feel it well up within you, growing your average sized human cock, like a balloon, feeling stretch to an unbelievable 2 ft long monstrosity, poking out from the loincloth underwear, only to be complemented by two grapefruit sized balls, pumping your penis full of cum. You keep moaning loudly, teetering on the edge of bursting forth, as you begin feeling stomach paigns, your bowels tightening, your asshole stretching involuntarily, as your body pushes out a inhumanly large pile of manure falling on the floor with large plop*. You should be disgusted but in the end, this is what tips you over the edge, the pleasure of pushing out the big pile of shit was one of the greatest things you've ever felt, causing you to spray thick yellow cum from your massive cock onto your new horse friend.
He just laughs, licking your cum off his lips. You realize what just happened, looking down at the pile of thick, brown shit, stinking up the room. "Don't worry," he chimes in, "you'll get used to it." You catch a glimpse of his overalls, only just now noticing, the brown stains all over the lower half and the smell of shit wafting from your new friend. "Especially once you fully change."
You try to open your mouth, but all you manage to do is unleash another volley of scat onto the carpet, another loud moan escaping your mouth as you do. "Don't worry, you'll enjoy being a permanent resident at this zoo with me," he says, his own cock tenting through his overalls as he does, awaiting your full transformation.