You can read a rather long story called ‘Roger Meyrick’s Ride at red4.co.uk/Folklore/trevelyan/snow2sea/rogermeyrick.htm
in which he finds himself riding a very strange and swift “horse”*, with the monumental and rationalising cop-out that it was all down to the effects of the local beer.
The story is part of ‘From Snowdon to the sea’ by Marie Trevelyan (1894).
*which turns out to be the cromlech.