The stallion that had formly been her son needed no further encouragement, he moved forward and postioned hinself over his new female. Postioning his member to enter this much smaller female was proving difficult however, as his meaty head poked around her cheeks. Frustated at the wait she reached back and guided it too her waiting entrance and with a series of small thrusts he began to try entering her.
Try as they might might however the cock was simply too large to fit inside, just as she was going to give up in frustation however something changed, a burning sensation began in her vagina and she felt it begin to slowly pulsate, moaning she wet herself as her vagina began to slowly expand and darken as her daughter's had only minutes before. Horrified she realised the spell was somehow being passed onto her, she had to get away, but it was too late and with a final thrust the stallion was finally able to penetrate her, driving the wind from her lungs as he began to fuck her.
"No no no! stop!" She moaned, but the stallion paid no heed as he bred her, her vagina now a mare's his cock filled her impossibly deep, and the changes began to spread throughout the rest of her body. Her already ample rear further swelled in size, a tail beginning to grow from the base of her spine. Her arms and legs began to lengthen, as she scrambled to try to get away she watched as her hands hardened into hooves before her eyes.
Moaning her torso stretched forward, growing with each passing second, her breasts bounced with each thrust the stallion made, begin to darken like her vagina as they slowly migrated to between her legs. Soon only a mares teats swung beneath her, swaying in unison with her mate's balls. Resigned to her fate she fully let lust consume her, whinnying in pleasure as she felt her face begin to stretch forward.
Now steadied on four horse legs the soon to be mare began to match the thrusts of her stallion, fine hairs were rapdily coating her body as her her neck extended, her hair forming a mane down its length. As her ears settled into place on top of her head her face finshed morphing into a muzzle and now it was a mare not a woman being bred.
Finally cumming she felt her humanity slip away as she she was pumped full of virile seed, her vagina greedily undulating as it milked him dry. Satisfied with her breeding her mate withdrew with a sloppy pop, he had bred two females now, but he would need more to grow his herd. With a whinny he led his two females from the yard, nostrils flared he already had the scent of his next mate.