You might be able to haul a horse in the back of a toy hauler, but how would you secure it? In a horse trailer, they are built stalls designed for horses, usually narrow, where the horses have a certain element of support and confinement. You'd have to build such a configuration in the back of your toy hauler. It might be easier to just stick with a horse trailer with living quarters.
As far as your question, yes, there are models "out there" with divided garage from living quarters with wall. Some wall are even designed to unlatch and swing completely out of the way.
Personally, I'd stick with a toy hauler for your toys, and let your parents stick with a horse trailer. I think it's probably safer for everyone and for the horses. There's nothing wrong each family having their own privacy you know!