I'm only an amateur in these matters, so hopefully someone who is professional, will drop by with a more educated opinion.
I do think it's a curious stone, all the same. It is a big stone for a field wall - perhaps it was there beforehand, and was later incorporated into the field wall.
It is very unlikely that the stone ended up that way naturally.
It's a bit difficult to see any remains of the wall - it looks more like a scattered selection of stones. If it is close to a field boundary, it could be the result of field stone clearance.
Maybe you could take a picture or two of the wider context. This would help to see if the associated stones are part of a wall or something else.
More often than not, stones cleared from a field will have plough marks - these can easily be misinterpreted as I should know
I'm sure it would be worthwhile to have a closer look to see if there is any sign of the hand of man, especially signs of shaping or boring - these might not be immediately obvious, and could need a bit of close scrutiny.
You have an eye for more than the 2 dimensional landscape now, I'll bet
Looking forward to more pictures.