"I just want my dick changed into a horse dick please." You say to her. "Great! Then we can continue with the procedure." She says back to you. She preps the syringe, then looks at you and says "OK you'll feel a little poke in 1, 2..." And before she got to three, she stuck the needle into your belly and pressed the plunger in. The tan colored fluid flowed out of the syringe into your body, leaving a tingling sensation under your skin.
Immediately, a warmth began building in your modestly sized cock, and it began tingling as well. As you watched in amazement, it simply began STRETCHING as if you were getting hard, but you were already throbbingly erect. Your dick grew inch after inch; as it grew, it began darkening in color, turning a deep black with pink splotches marking the length of the shaft. As it grew past a foot in length, the head swelled past a mushroom shape into a blunt tip that flared near the end, throbbing and straining as it changed.
Your balls also changed shape. They ached and churned as they swelled in their sack, the skin becoming leathery and they grew past the size of tennis balls. You reached a hand down to grip them, brushing past your now-huge horsecock, already past two feet long. As you feel them in your hand, you shift from one to the other, each filling your whole palm and churning and boiling with new horselike seed.
The wild stretching sensation in your cock stopped as it neared 2 and a half feet long, and a small drop of precum started pooling in the blunt head of your new and improved member.