Mating the mares would establish his leadership, but in order to maintain leadership that way, he would need to mate with every mare minus Thunder regularly. The number of mares and frequency of mating would slow them down considerably. No, he needed his mares to carry his offspring. Then they would follow without question. Luckily, Gelvin's favorite fertility spell, the one he had been using, had a quirk to it. It allowed for a stallion to impregnate an already pregnant mare with an additional foal. It wasn't a guarantee to work, so Gelvin had to cast it on himself and his mares and then mate until they were carrying his foal.
Gelvin eagerly mounted Rain first. She happily took him, hungry for his seed in her womb. He emptied into her and scanned her. She was pregnant with twins now. One foal was Thunder's before she was turned to a mare, and the other was Gelvin's. Thrilled with his success, Gelvin made his way through the other mares, leaving out Thunder, already pregnant with his twin foals.
Thanks to the charm, the new mares took Gelvin as eagerly as Rain had, neighing with pleasure as he put his seed into them, making them carry a second foal. Finally, Gelvin got to the last mare, a beautiful palomino named Glory. To his surprise, she already carried twins. He would have to keep her in line by mating with her, not breeding her, then.
Gelvin has just mounted Glory when he realized something that made him pause.