The neverending story

Your body is wracked with pain. You writhe on the floor in agony as your muscles and bones reshape, tearing your clothes.
Short bristling chestnut hairs are growing all over your skin, and you can see your nose extending out before your eyes. Your fingers and toes are shrinking into your hands and feet and hardening. Your neck is growing longer, and stronger, and your back is stretching, as your hips rearrange themselves, forcing you to remain bent over.
You are a horse.
Suddenly, the wall before you opens outward, revealing a clear sunlit day. The claustrophobia caused by confinement makes you bolt, into the large corral where the chrome ship now rests.
As soon as you're clear, the ship takes off into the blue sky and is gone from sight.
Your trying to gain control of the strangeness, and alienation, when you hear a sound behind you.
Turning quickly you see a man coming towards you through an open gate. His clothing seems medieval, like a farmer, or perhaps a peasant.
"Well. A new one eh? I guess I'd best explain your situation, and get your trained, hadn't I."