Well... flail-like weapons are known in both Europe and Asia as peasant weapons. Not many armies used them, apparently because using them really well requires a lot of training. The only European army to use them in significant numbers were the Hussites in the Hussite wars (an early war of religion). The Hussities might be a model for Flagellants, as they were zealots who fought w/ two-handed flails and whose only armor was padded shirts and leggings.
I once asked the
Higgins Amory Museum folks about what we know of flails, and the answer is "Not much.". Most of European martial arts was with swords, knives, or pole weapons, and the one old book that talks about flails seems to have been written by a guy who never actually saw one. (See "peasant weapon".)