There's a back story. But it's too long for one of my customary texts. In short --

A young thief is being hunted, and falls in with a trader who also dabbles as a witch. She's in a pickle of her own, having promised a tame unicorn to a customer while she was drunk. She has no such thing of course. The reason they're so valuable is because unicorns can't be trained. The fugitive thief gives her an idea. He doesn't like the proposal at all, but sees the sense of it. If she can cast the spell to turn him into a unicorn, he can sneak past the town gate with her. The drawback is that he has no desire to spend the rest of his life in some rich noble's private zoo. The merchant-witch says that as long as she can show him to the customer now, it needn't be him she delivers in the long run. If she finds a substitute -- willing or not -- she'll change him back. In the meantime, he's outside the gate and free. He agrees.

Of course, once outside, she talks him into remaining a unicorn colt. After all, the law will search for him in the country side once they realize he isn't in the town. Reluctantly, he agrees with her reasoning.

Chapter by chapter, the merchant-witch finds reasons why she shouldn't or can't restore the young thief to his human form just yet. She strings him on until they've made the entire journey to the customer's estate, umpty-ump weeks travel & adventures later.

At this point he's pretty used to being a four-footed critter. The fact is, its easier than working for a living... or stealing. Not only that, but pretty girls seem mesmerized by him, throwing themselves all over the randy little unicorn. He still wants to be human, but he is no longer in the least hurry. Before he realizes it, the witch is talking him into staying a unicorn a while longer, even though it means belonging to an owner. She makes more promises that she'll find someone to take his place just as soon as she can. In the meantime, she coos, he'll be spoiled rotten.

But they're only more lies. Before he knows it he's lured up on a slab of stone and can't move. He's turning to stone. It was never a tame unicorn she sold... she sold a marble unicorn, and tricking one into delivering itself was far easier than carrying it.