Will the goats and horses form a herd? My horses are super protective of everything they like, so I hope they would...
Our horses and goats have always coexhisted really well...I have been slowly introducing my goats and horses, here, because I would like to eventually turn everyone out together during the daylight hours in the summer. So far aside from alot of curiosity, they have been good...no one's been mauled, tehehe. You might occasionally get one horse that likes to chase and pick up a goat and shake it (which can of course kill the goat, given the size of a horse's jaws!), so if your horses have never been around goats before, introduce them slowly, and don't simply bring them home and toss them in your pasture! Pen them near the horses, so they can sniff, and look at each other, but so that the goats will remain safe from over curiosity, or possible chasing/attacking. Once the horses don't really pay a whole lot of attention to the goat's comings and goings, and antics in their pen, then start turning them out for short periods of time with the horses...it's really good to start out with only one horse in the pen with the goats, but if that's not possible, you just gotta be watchful of the introduction, and be ready to shoo any naughty horses away and rescue the goats!
Generally, though, horses, goats, sheep, etc, can coexist really nicely, and goats can be loads of fun to watch when they get playing and bouncing around.