"I've waited a long time for a human to come across this place, and finally I've got my wish"
"As you can easily tell, I'm no ordinary horse, I am much larger than a normal horse, a good 5 feet long, and 3 feet taller, but thats the least of my differences from your everyday horse, in fact I differ a great deal from different horses internally...."
With that, an second opening appears in front of you, and the horses colon contracts, pushing you in head fist.
Your body is spread out, your arms are at your sides, and you are pushed into a position in which you are basically lying down.
As your feet enter the chamber last, the opening closes behind you and disappears, you hear the horses muffled grunt.
The muscles around you contract, wrapping around your body tightly, something odd wrapping around your penis....
"Well human, you're exactly where I want you, see human, I have a special organ inside of me, capable of concealing, and keeping a human alive for practically forever. I can bring my manure into the chamber next to your mouth for your sustenance to keep you nourished, and your waste out to keep the organ clean for you, and don't worry, my manure is very nutritious, and also has some fun properties to keep you erect constantly. Finally, the most fun part of this organ is what I've placed over your genitals, it functions more or less as an internal vagina of sorts, and whenever I fell like it, I can use it to have sex with you, whether you want to or not."
You're stunned for a moment at all that has transpired in the short while since you've first seen the horse.
"You can't do this!" you plead
"Can and have human, I suggest you learn to enjoy it, or this will be a very unpleasant experience for you indeed.
"You will be my permanent mate and plaything, and I'll be sure to make you enjoy it"
With that, the horse trots back to her stall and begins to eat hay.