@rax the passed S test means you've got your horse under control - you don't need to worry about it immediately throwing you off. It might still panic if e.g. the wyvern tries to take a bite out of it though.

One note is that the terrain here has a lot of bare rock, loose stone and steep slopes, which is why I've assumed everyone's leading their horses rather than riding them. A horse going fast over this ground could possibly do itself an injury.