I think that dogs try to bite off more than they can chew.... And end up getting nipped back. Just like it isn't wise for dogs to mess with cats, Rover clearly forgot that when he antagonizes things that are bigger than him, he needs to make sure he is assuredly faster than them beforehand. Also he might want to go over social norms for greetings, because it seems like the horse doesn't appreciate how dogs say hello, quite the contrary.
Need more horse? These horse pics are pretty nice to look at, but not as majestic as these golden horses, nor as funny as these horse memes. I guess you can say balance is the key to good internet horsing, just like real horses.