Judging the size and were you found it I say a Wagon wheel snow breaker, sort of like the semi trucks today have, breaks up the snow so it doesn't pack onto the wheels. Montana had lots of them, they were usually forged of iron and quickly so like snow flakes no two are alike.
This is just my opinion I'm not an expert but I see similar things like it at old farm auctions here. Home made fence post tampers look similar also but that would make it newer. If there was a sign of old fencing around the trail you found it on or the trail was wide enough for house and carrage could give a better clue.