This nice bit of rock has an interestingly pagan-sounding bit of folklore, summarised by Sir William Fraser (in 'The chiefs of Colquhoun and their country', 1869):
Under Benvoirlich there lies on the roadside near Lochlomond a stone of large dimensions, called Clachan Tarrow or the Bull Stone. The history of this stone as told by tradition is, that it was rolled down the mountain in a desperate struggle between two infuriated bulls. Forty years ago a pulpit was cut out on the side of the stone fronting the road, from which the minister of the parish might occasionally preach to those of his parishioners who lived in this remote district, which is ten miles distant from the parish church.
Hmm. You'd think it'd be quicker just to stand on a box. But JM Briscoe's photo on geograph does show it to be an impressive backdrop for ecclesiastical ranting. More details here on the Arrochar Parish Church website (but for goodness sake turn off your speakers. You have been warned).
More about the bulls is found in AD Lacaille's 'Ardlui Megaliths and their Associations' (PSAS 63, 1928/9):
The strong flavour of mythology in the Gaelic name, "Clach nan Tairbh," for the Pulpit Rock, is accounted for in the tradition of the Red Bull of England and the Black Bull of Scotland meeting in mortal combat on Ben Vorlich. So terrific was the contest that the rock on which they fought became detached by reason of the shocks it was subjected to by the onslaughts of the infuriated animals, and finally it slipped down the slope of the mountain to rest permanently in its present situation. Victory, we learn, was with the northern bull which, with its crooked horn, dispatched its rival (Campbell, Popular Tales of the West Highlands). The story ends with the statement that Clach nan Tairbh "is the largest boulder in the three realms" - an indication that the legend associated with the place may go back tothe time when this country was still divided up into the three kingdoms of Strathclyde, Dalriada, and Pictland.
Like me, the author clings desperately to ideas of the rock's ancient significance: "To reply to a speculative inquiry as to why a more convenient spot, such as a house, should not have been the place for meetings of a religious character after the fall of the ancient church, consideration must be given to some traditional significance borne by the huge boulder to the minds of the inhabitants of the locality - a significance, moreover, which had its origin in remote antiquity." yeah yeah.