Like the iron maiden, the flail is an invention of a later time, or so says the Public Medievalist:

The Curious Case of the Weapon that Didn’t Exist

Just about everybody interested in the Middle Ages, who has played Dungeons and Dragons, or read historical and fantasy novels knows what a military flail is. It’s one of these:

A military flail is a medieval weapon consisting of a short handle attached to a chain, at the end of which is a metal ball. This is not to be confused with a two-handed variant, often also called a flail, which derives from the threshing implement of the same name. Varieties of the one-handed version have multiple chains or spiked heads. They have appeared in a range of medieval movies and books, and they are held in the collections of museums like the Metropolitan Museum of Art.

Only problem is: they never existed.

Despite the weapon’s popularity in pop cultural depictions of the Middle Ages, the flail was almost certainly an invention of the imaginations of later people.

More at the link, including commentary from Kelly DeVries.