Derrick was disturbed to be turned on by the males - he was not ready to try changing sex. Species was enough. And if he was going to try a different form, he might as well go for the biggest. The cow was ready, tail flagged and her opening huge and slick. Way too big for a puny human to fill, but that was temporary. He had to stand on tiptoe to enter, and felt totally inadequate at first, but felt himself start to enlarge. Muscles swelled, bones lengthened and changed, and most importantly, he felt himself swell to bull's proportions inside the female.
As he began to fit his pleasure increased, and he bellowed as intensity overwhelmed thought for a while. Animal instincts were in control, and Derrick completed the mating as a bull. He slid off, shaking a heavy head that felt thick and dull and very, very satisfied. It took a minute for senses to clear enough for him to realize he was on four hooves. So this was what changing form was like. He felt so massive, so powerful! He liked this! And there was another cow waiting. Derrick plodded towards the waiting female, letting the bull's mentality take control again. There was need on both parts, and he found it even easier to settle into the animal mindset.