I want to see that !!! It wants to be free . Do you have more than one horse ? Is there room to run ? Is it lonely ? Is it looking for food ? Is there an apple tree on the other side of the fence ? Horses need company with other horses . They love apple trees . They need to run . If you can't provide that . It may want to escape . If it has all of that . God bless you . You have to provide it with a fence that will keep it in like the ones already mentioned . A horse that can and wants to crawl is a good rodeo horse ! Honestly !!
Obviously your fence is lacking bite. Do you have a ground rod? Is the box big enough to handle your fencing? If yes to all of that, check to make sure it isn't shorting out someplace. Then give serious consideration to a better box