Horses sometimes can be pretty dangerous animals, especially because they can harm their owner. Usually, these animals are very gentle and tender, but when they feel that someone is about to hurt them, they become violent.
The horse is devoted to its owner especially when he is loved and taken care of, and when its owner offers him protection, the horse feels that he must offer protection and security in return.
We can see this in the video also. Even if the cows are not aggressive the horse keeps them at a distance until his owner finishes his work.