Near the castle (opposite of the stable) you are able to get under the map, the ground is open.
And I'm not shure if the horse is supposed to follow, but it doesn't.
great game! keep up the good work.
as for the horse.
The horse is currently disabled for being a naughty bad horse :< He broke a bunch of people's spines and forced them to crawl slowly in the ground and kicked their things sky high into the air and then spat on them as he decided to soar away and scoff at our ideas of physics and gravity. It's planned to eventually come back, I think... when its not so psychopathic.