Different horses can have different personality. Some horses are more playful than others. Some horses are more submissive than others. Some horses are very wild.
In the case you mentioned, I believe the horse was just trying to play. After all, he didn't get to watch TV, he didn't get to use the internet. They can get bored lol. Now all he can do is to knock that hose loose. Spraying water might look fun to him, especially when it spray at a person.
