A fine start
I see this submission merely as a demonstration of what could be done in the future, and from this perspective it is a very good one. Riding your stallion was fun, especially riding it at free will in three dimensions. Changing the stallion surprisingly was great, too.
The graphics could be a bit better, the rider for example could be better animated. The surroundings were fine, though.
Oh, I missed but one feature: increasing and decreasing your stallion's speed. It would provide even more interactivity.
All in all a fine demo and keep up the work so that we may see a full game in the (maybe even near) future.
Alarion