I wonder if it was once known in the singular, but it got changed over the years as it lost its' meaning?

G x

"The Devil's Quoits, a henge monument c. 1,200 yd. south-west of Stanton Harcourt, (fn. 21) was used throughout the late Neolithic and Beaker periods, and comprised an embanked stone circle c. 280 ft. across, composed of 30 or more stones; it was mostly destroyed in the Middle Ages to make way for cultivation, some of the stones being broken and possibly re-used in Stanton Harcourt church. One of the three remaining stones was used as a bridge in the late 17th century and again in the late 19th or early 20th, but was replaced following protests from the Harcourts and local people. Only one of the stones was in situ in 1940 when the airfield was built over the site, later used for gravel workings, and the stones were buried; in 1988 there were plans to restore the monument incorporating the buried stones. (fn. 22) A tradition that the Devil hurled the stones from Wytham hill during a game of quoits was current by the late 17th century; (fn. 23) the idea that the monument commemorated a Saxon victory at Bampton in 614 originated in the late 18th"

From: 'Stanton Harcourt: Introduction', A History of the County of Oxford: Volume 12: Wootton Hundred (South) including Woodstock (1990), pp. 267-274. URL: http://www.british-history.ac.uk/report.aspx?compid=6925 .

Surely the quoits are the stones?! A quoit can be a flattish sort of stone, can't it?