It's called a "bar shoe, and there are several reasons one of those shoes is used. It's a corrective shoe, and can be designed to put pressure on the frog, which would be an attempt to cure contracted heels, which is a very painful condition for the horse. Also, if the hoof wall is broken up, and you can only get one sound nail on one side of the foot, then say with 4 sound nails on one side, the bar shoe helps hold it on the foot. However, usually there are some toe and side clips involved when that is the problem. The toe on your shoe is worn so badly that if there was a clip, it's gone now. It's a shoe for a front foot, and with the wear on the toe off to the side, depending on the foot the shoe was on, the horse was either paddling or interfering when it walked or trotted. Anyhow, it is a corrective shoe still used today, and truthfully, very hard to date. Anyhow, dating other finds in the area would be the best bet.