Although he let the horse instincts and desires take control, Derrick was still fully aware of his past and identity. But this form felt so good - like the humanoid worker, but even more relaxed and accepting. The draft horse was content, and the more Derrick let himself drift into the animal mindset, the less he thought at all. By the time evening came and the animals were led into stalls for grooming and feeding, Derrick was for all intents and purposes a Clydesdale stallion.
Over the next few days the former boy was used to pull wagons and plows, his actual identity forgotten even by the staff. He performed flawlessly, to the point that he became a favorite when assignments were made. Someone crafted a sign that stated his name to be 'Bubba', which he quickly started to answer to. In the back of his mind, the animal vaguely being called something else, but that name was no longer important. He mated with a few willing mares, which cemented his identity as a horse even more, not to mention starting some offspring that would be highly intelligent.
Derrick became aware suddenly while pulling a wagon. The work was easy, automatic, and he felt very satisfied. But he wasn't a horse before. Memories of his human past resurfaced and he realized his two week 'vacation' was over. It was time to let his Masters know he was a guest so they could return him to his family. Except - he liked this even more than being a centaur's groom. Life was easy and natural, with no thoughts at all needed. No pressure, no responsibility. Did he really want to go back to being human?