Something reaches inside your brain and, probing, finds every detail about your idea of a dream horse. The breed, the color, the look, the age, the gender... even hoof and mane length, and tail thickness.
Then the searching claw recoils and you feel something happening:
First, you are expanding. Expanding to an enormous size, or at least what seems like it. You can feel yourself being blown up like a balloon.
Your neck lengthens and thickens. Your face is stretched, and you an feel your nose flattening and widening, your eyes pulling apart. Your ears grow much taller and move from the side of your head to the top of your head.
Then your arms and legs are stretched longer, unbelievably longer. You feel a shot of pain as your joints rebend themselves in a different direction. Your fingers are pulled together and melded into a hard form.
As a grand finale, hair shoots out of your new body. It feels odd, like somebody is pulling on your skin, but it doesn't really hurt... at least, not THAT much.
Then, the mind prober comes back, this time taking every past memory out of your mind and replacing it with blank, dark space. You see all you have ever known pass before you, and then it is gone, swept away, never to return.
The world stafts to swirl and you see a mixture of colors flashing before your eyes. You scream, and it comes out a high pitched, howling shreak that trails off at the end, like a bugle. Then you hit the earth, and everything goes black.