Although I am sure it is possible to get a horse to mount you it is not advisable. Not only is bestiality illegal just about everywhere in the world but it is also considered cruelty to animals. Horses are not built like humans nor do they mate like humans. A horse can easily kill a human during mating. Most of the videos you can find that feature horses and humans are video edits and are not real.
Although it is possible to get a stallion to mount a human you do not want to do this. Humans and stallions mate very differently and are drasticly different in size. Stallions also become extremely violent when they are aroused and can kill you or at the very least seriously injure you.