In an alternate timeline in which Khan Noonien Singh won the Eugenics Wars and Humanity underwent genetic engineering en masse, a Princeps was a command level position, likely equivalent to Captain held by an Augment in the Earthfleet of the Children of Khan.
Julian Bashir held the position of Princeps of the Earthfleet warship Defiance. Ethan Locken held the same rank, as did William T. Riker of the warship Excalibur. In 2376, Jean-Luc Picard, the commanding officer of the warship Illustrious, held the position of Princeps Senior.
A Princeps usually had an Optio as his second in command. (DS9 - Myriad Universes novel: Seeds of Dissent)