Not spark plug related. Way too big, and it looks to me like the ends are made of concrete. You can see the aggregate exposed by the chipping, especially on the large end.

I'm going to go with a post on which a chain link fence gate swings. This would sit in a hole in a concrete pad, the gate would have a metal loop that goes over it near the ground, and a second similar hinge piece on top that affixes to the fence.