Once you've finished getting undressed, you saw the unicorn's horn shine a bit, as you felt your own testicles melt away, while your penis became completely erect. You had no idea what was happening, until your testicles fell out of the skin of your penis, and got replaced with a small opening, revealing a pussy under your penis.
You could feel a womb forming inside you, as you find you were able to feel more attracted to more unicorns. "What happened to my balls?" You ask. "Did you neuter me?" "No, all stallion unicorns are actually herms," the unicorn answers. "That's why you have a pussy under your penis now. Okay, now lay on all fours for a second, so we can begin." You do what the unicorn says, and get on all fours. Of course, you didn't know what the unicorn was going to do, until you started feeling pain in your hole.