The modern flag of Belgium takes its colors from Brabant's coat of arms: a lion or (a golden lion) armed and langued gules (with red claws and tongue) as a primary heraldic charge on a black field.
Probably first used by Count Lambert I of Louvain (ruled 1003-1015), the lion is documented in a 1306 town's seal of Kerpen, together with the red lion of Limburg.