Who were medieval flails used by?

they were used by knights on horseback and foot soldiers

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Answer: Flails were originally agricultural implements used by peasant field workers and dating back to Roman times, used in farming contexts right up to the 19th century. They were threshing tools, meaning that their primary use was for "threshing" or "thrashing" the sheaves of corn (wheat, barley, rye, oats, millet and darnel); this was done in the cold winter months in a threshing barn by teams of men stripped to their underwear, since the work was long, hot and dusty.

Threshing flails had long handles of ash wood and "beaters" of heavier blackthorn or some similar wood. The two sections were at first joined by leather straps, later with rings of metal.

Like many farm tools, the flail was adopted as a military weapon, being simply shortened in both handle and beater and having metal spikes or studs added. But it was a weapon which took great skill to use, since it could be just as dangerous for the user and his comrades as it was for an enemy. For that reason very few were actually used in combat on the battlefield - they were most often used in single combat or in tournaments.

The link below takes you to images of medieval flails in their original, agricultural form: