Getting Apollo ready for the Centauride is not quite the same as regular horse training. While he does need to know and be able to do all the same things a trail horse or a pleasure horse, he also has to be accustomed to things that most horses don't have to deal with.
Llamas, cows, and other animals
Kids and crowds, including baby strollers
Traffic, including honking, fast cars with little space, and semi trucks
He is also learning how to wear hobbles, picket-line, and other backcountry skills.
Fortunately, he already is used to loose dogs running at him and under his feet, as well as cats and chickens. No, chickens aren't scary animals, but try to tell that to a horse that's never seen one!