This rubbing stone for animals stuff is shit. The stone's there and animals scratch themselves on it. A tree would do. In the catalogue of standing stone shapes this is the one that's called Fin. (Not that there is a catalogue). These are the kind of things that used to be placed, often in pairs, as outliers, to somewhere else, to indicate a setting or rising. But, please, if you go back for more pictures measure the hole diameter, in inches. It should be possible, using the stone, to find other lost bits in that landscape. You'd need permission from the farmer, of course, and you'd want to tell him about the stone. Then the county archaeology unit. It's worth sitting on it for a while if you're looking about for more stuff - not literally, of course. From the pictures it looks like sandstone, with a lot, possibly a seam of quartz. But photographs can be deceptive. The clincher will be the diameter of the hole. Correctly they're called tolmen - but are very controversial.