Roman helmets had brushes or plumes of horsehair. It seemed to have been a purely astetic addition, with little practical use. However it was the continuation of a type of helmet known as the Illyrian Helmet which had horse hair brushes. read more
Only the helmets of roman generals, centurions, Praetorian Guard and high ranked officers had what you call a brush. The common legionaire hadn't such a thing on his helmet. It was of course, along with the red cloak, used a a sign of nobility, prestige or high rank. read more