pretender

A pretender is somebody who claims a title they may have no right to. The Old Pretender, a pretender to the throne, dismiss him as a pretender.

In history it is a technical word and not necessarily an insult: a pretender is a claimant whose claim was not recognised, and whether that made them wrong depends on who won. The Old and Young Pretender are the Jacobite claimants of the eighteenth century.