OP, none of the answers here are satisfactory. Please ask the rangers and update this post because this is fascinating.

Google Lens wasn’t helpful, it compares it to things like drains and fences built around old markers (it specifically thought it was a Mason-Dixon line marker which is obviously incorrect). But this is not protecting anything and not a drain.

My guess is, if it’s not something to tie a horse to, is maybe it’s something to put animal feed in? But that’s a wild guess, I don’t really think it’s right. Like shove a hay bake in. But you said only accessible by walking, so I really doubt it.

It looks like a park but it’s a municipal camping with a forested area for tents. I will ask the employees when I go back, but I’m afraid they may also not know. I’m also tempted to call and ask directly but this gives me a reason to make another cycling trip “in the north”.

I hope you’ll post what you learn as a new message in this community, so we all see it.