Low wood rail fence.

As much as I prefer wood fences to most other types, this fence is not tall enough for the size of the horses kept inside.

The top plank should be higher than the point where the horses' neck and chest meet, at minimum.

I like to see them higher, so that the horse has to LIFT its head to put it over the top rail. That way, if they push against it, they cannot throw their full weight into it like they could if it were chest-level.