http://www.publicmedievalist.com/curious-case-weapon-didnt-exist/ has an article about the flail - namely, how your one-handed ball and chain likely didn't exist in the medieval period but was artistic licence by illuminators, and then later people made examples of it. The two-handed agricultural flail with a short chain may well have been used as a weapon, but not the D&D-tastic spiky ball swung on a chain.
Really? Because I recall seeing one on the wall of a medieval castle during a tour that was not a replica...
also as I understand it the flail was useful because it could be whipped around a shield...
Most medieval castles we encounter are 17th century and later, or have been remodelled to those standards. Not to mention that the "inventory" we see in them is usually added much later than the days of it's actual use.
Sigh. I thought the Witch King's flail in RotK was a copy of a realistic weapon; I'm crushed. 
Roughly so.
All medieval castles, that survived until our 21st century, changed use and purpose several times. Their specific military purpose - withstanding the siege of a medieval army - was lost very early in that process, as was most of the equipment needed for that purpose.
Medieval weapons have typically come to us in the armories of cities or the collections of families. But separating the historical weapons from the decorative collector's items in them is a tricky job for experts.
Even in the late 19th and early 20th century there still was a vast market for decorative, medieval looking items: look for example at the Louis Marcy (Luigi Parmeggiani) fakes. Those understanding Italian can start with the Galleria Parmeggiani in Reggio Emilia (jhc.oxfordjournals.org/content/22/2/338.extract .
Cheers
Yeah, there are some problems with the historical attestation of the flail. May I note this?
youtube.com/watch?v=AGf7n7iUF_k
He doesn't seem to be able to push them back before C15th.
This drawing is interesting, as it shows a peasant troupe armed with flails: 
It is early 15th century, however - the subject being blind Jan Žižka leading the Hussite army.
Anyway, the English wiki article en.wikipedia.org/wiki/Flail_%28weapon%29 is well documented and puts Sturtevant's in context.
Cheers
The main problem is there is a lack of documentation of anything before the 15th century. Also versions of the flail can be found which were used to punish criminals or drive animals rather than use in warfare. On the other hand in time of crises anything that could be used as a weapon probably would be, similarly to how a nail gun or chainsaw would not be described as a weapon but certainly could be used as one if need be.
As to the arguments about how unwieldy the weapon is- at first, yes, and certainly it is handled very differently than a sword, but after some practice I had no problems wielding one safely...
Ahm.
We have lots of descriptions of fights, marches, battles, tactics, and even logistics of armies: so we can expect to find the use of common and typical weapons in the medieval literature.
But descriptions of technology are very rare: so we have to study weapons and armor found or passed down to understand the metallurgy and smithcraft used for them.
Cheers
AFAIK, a very small number of (ball&chain) flails HAVE been authenticated as medieval, while the vast majority are fakes or from later.
The most accepted answer i´ve seen is that they very probably existed but simply were not used much for the simple reason that they are complex to make and even far more so to use.
Modified threshers type of flails are also KNOWN to have been used, not just in Europe but S.Asia, E.Asia and Africa, long enough back in time for it to be essentially as close to proven that they existed in medieval time as you can get without going back in time and look yourself.
Followers of Peter the Hermit are stated to have used thresher flails in [large(?)] numbers in the 12th century.
And developments into the ball&chain type of flail happened in several areas, probably even independently over time, so the possibility that it may have happened slightly earlier than is known for certain?
Fairly likely at least. Most things were almost certainly developed earlier than we KNOW that they entered use after all. And anything that isn´t a showpiece nor a common item, the probability that they survive until our days is minimal.