Barber & Williams (1989) list it as the remains of a cromlech. They say Samuel Lewis‘ A Topographical Dictionary of Wales (1844, vol II, p.198) refers to a large stone several tons in weight, so nicely poised on three small upright stones as to vibrate on the slightest touch and upon being struck it sounded like a bell.
They go on to say it was blown up with gunpowder by village inhabitants, and that Archaeologia Cambrensis in 1974 list two stones both 6ft high.
Clearly, if it was a cromlech then either one of the stones has been moved, or else the cromlech had an outlier. I’d suggest that if either’s been moved it’d be the southern one – the stone in the middle of a field is not as likely to have been moved as the one by the downhill field boundary.