Ideally, Gelvin would breed his mares. But the only way he could do so would be to make them carry twins, with one foal being his. He didn't want to do that; with two different fathers to the foals, it would be obvious he had impregnated their mothers, forbidden for Horse Witches under Witch Law. He could do the next best thing, however.
After introductions, Gelvin mounted the white mare named Iris immediately, without warning. Iris whinnied in shock, but the soothing charm was still in effect, so she settled quickly. She neighed happily as Gelvin emptied his seed into her. He dismounted and nuzzles her. Iris meekly and obediently nuzzled him back. It had worked. Iris viewed him as the herd leader and likely her mate.
The other mares figured out what was going on, but didn't do anything other than let out an initial neigh of surprise as Gelvin penetrated them. Rain and Thunder, still under the effects of breeding magic, interrupted Gelvin before he could finish with the new mares, insisting that he mate with them. Gelvin couldn't turn them down or risk losing some authority, so he mounted them and released his seed as quickly as possible so he could reach his last mare before the charm wore off.
Finally, Gelvin approached the final mare: Nightingale. She was stunning, with a sweet voice. Of all the mares, Gelvin wanted her as a mate the most. She looked at him dumbly, swishing her tail. Gelvin nuzzled her and then went around to her back. He silently prayed the charm would hold as he got up, placing his front legs on her back and aimed his cock at her pussy.