You have a luxurious dinner with the lady, though you notice there's a strong focus on vegetables. You wonder about this and ask her:"Do you happen to be a vegetarian? I see no meat..." She smiles and says "oh, I thought you'd like it. Most people I've had in here claim they started to like greens more after a meal like this, even." You can't deny you're starting to like the taste of it more and more, even though your insides feel strange...as if your intestines are shifting around. You pay no attention to it and finish the meal, after which the lady invites you to a tour of the mansion. "Oh, do stay...this house is a bit far away from any other place, and there'll be a ride here soon. Much more comfortable method of travel." You thank the lady and agree, and the first place you visit is the house's stables.
You take a surprised look at the empty stalls. "There's no horses around?", you note curiously. "Oh, there will be soon. Check out the insides in the meantime." You step inside one of the pens and one the walls there is various riding gear to put on a horse. Amazed as you are by this spectacle though, you are starting to feel a bit sweaty, not noticing yet shiny brown hairs are forming all over your skin. "And now", the lady smiles, "to get you into the right position for this." She takes the head gear off the wall and puts it on your head, shoving the bit into your mouth and tying a rope attached to it onto a pole before you can even react. You try to ask her in a muffled voice what the hell is going on, but your face hurts too much to talk. Your teeth become at least twice as big, your entire face simultaneously stretching out and ending in enlarged nostrils. You look at your own brown-haired muzzle, not able to see the patch of white on your forehead, and let out a surprised whinny.
"It's gonna be a lot more comfortable traveling this way for you", the lady says, "and for me as well. I need a good traveling horse to ride to various places. Don't worry, you'll receive the best care possible." You come to your senses and try to take the headgear off, but your fingers have grown too rigid to do anything. In fact, they are fusing and turning black, flat at the ends except for the rims on your new forehooves. She starts undressing you at the same time, "would be such a mess to have all your clothing torn up...", and as she does so you can see that your entire body is now covered in the same brown hair. The fact that your toes feel so stiff is adequately explained by the fact that they have become perfect hind hooves. Struggling to stand, you drop into a clumsy four-hooved position, crouching more than anything else.
Then you feel yourself rising. Your arms and legs gain length until they are the same length, and your body feels as if it's suddenly bloating up. In fact, it is gaining muscle and fat all over, and makes your overall head height slightly above the lady who is not standing outside the stall. Your head has entirely changed down to the eyes, ears and mane, and she gives it a friendly stroking to which you vehemently protest with an angry whinny - to no avail. Almost complete, your hindquarters inflates into a proper-sized horse butt and slightly above it, flesh and spine start growing out and covering with a large bush of silky black hair into a full-fledged horse tail.
In the narrow space allotted to you, you try hoofing at the front of the stall in an attempt to open it, but the lady has made very sure her new strong riding horse will not get away. Unless she wishes it.