You saw dark fur starting to creep up your body as your hands and feet started to meld together and harden. You tried to cry out but only a loud whinny came out as your body started to transform and bulked, your clothes straining against them, you hadn't expected this to happen, and you had no idea what was going on.
You ran forward, stumbling over your transforming feet as you made your way to the mirror and let out a loud whinny of surprise. Your body was already half covered with black fur and you managed to force your pants off and gasped in surprise to see that your previously medium sized cock was growing massive, and more horse like.
You were turning into a horse, now everything was starting to make sense. You tried to get out of the rest of your clothes, but they just tore apart as your chest barreled out. Your face pushed out into a muzzle as your ears migrated to the top of your head. You felt your spine and legs adjust themselves as you fell forward onto all fours, your neck getting longer as your new mane formed over it.
You neighed as you shook your head, your cock retreating back into a sheath as your transformation finished, you were a full stallion and you felt the same urges as one, you wanted a mare to mate with, and you knew that one would be here soon.
You just had to wait for her.