This is a hefty (but sadly fallen) stone - the RCAHMS record says it is 4m long x 1.4m x 0.5m. It has understandably been known as the 'Great Stane of Pendreigh'. It was (supposedly) placed here as a monument to Wallace's victories - see the folklore of the Seven Stanes at Wester Biggs.
I'm not sure if the following story also refers to it. http://www.archive.org/stream/dunblanetraditio00montuoft#page/19
"If" the 'Loup o' Pendreigh' is the bridge over the Wharry burn, the stone is indeed maybe a mile away. But it's hardly in a 'haugh' as that suggests a meadow next to the stream, and the stone is up some way from it? It is about the Shepherd of Braco who (trained in the use of the broad-sword since infancy no less) manages to despatch an unlikely number of dragoons, who are then buried at the stone.