The Centurion in an untrainable heavy cavalry unit in Age of Empires II HD: The Forgotten. It makes appearance in several campaign scenarios and is available in the Scenario Editor. It's appearance is strongly reminiscent of a Cataphract, and the stats are exactly the ones of an Elite Cataphract; so essentially, a Centurion is no more than a slightly different looking Elite Cataphract.
Centurion was a rank in the Roman army, the commander of a Centuria (100 men).
The Centurion is inaccurately portrayed as a heavy cavalry wearing a Chalcidian Helmet despite being a Roman infantry officer wearing a Galea; it would be more accurate to call the Centurion an Equite, Legate, or Clibannari.