Goats are usually very friendly animals and can even calm a human down.
I'm not exactly sure why goats keep horses calm but they do make good companions for horses and even dogs.
I had a dog that was in the same fence with the goats and he would even try to get up to the branches of the leaves himself and act like the goat.
When one of the goats passed away the dog was very upset but the other goats seemed to calm him down.
Goats are wonderful animals and are good therapy animals for both humans and horses as well as dogs too.
Goats get along very good with horses and some dogs as well.
Goats will also sometimes play with the horses which also helps them to calm down which is one of the reasons goats keep horses calm.