RonB wrote:

Need a little more detail on how the fence is going to be built.
It almost sounds like the 1x6's are being put into the slotted
holes.

Yes.

Is this a solid panel privacy fence or a post and rail fence?

The posts are going to be 8, or maybe 10 feet long, set 3 or 4 feet into
the ground (where the frost line is in this area). This would put the
top of the posts about 5 or 6 feet above ground. Posts would be set 8
or 10 feet apart.

The three 6" boards would be spaced out, probably starting 2 feet above
ground, with 1 or 1.5 feet between boards.

The purpose of the fence is to contain a few horses within a certain
area. Presumably horses rarely attempt to jump over fences that are 5
to 6 feet in height.