He rolled his eyes up, but as he felt that muscle begin to suck on him as he thrusted, he felt what once was a pressure, now had give. Something in his cock was giving to the incessant sucking of the horse pussy. He did not know that he had a sheath like a stallion, but it would take being inside a mare to protrude and begin to stretch not only his urethra, but her pussy. He gasped and his eyes rolled up as a curious sensation began to flood him as he began to thrust harder and harder against her.
His stallion sheath began to spread her and fill her completely. He grabbed her haunches and began to buck no longer having all the room he had before. He panted and rolled his eyes up grunting softly to her.