Pope Peter II is a hypothetical papal name and, in recent times, a common name for sedevacantist group leaders styling themselves as popes.
Pope Peter II Wikipedia
Out of respect for the Apostle Saint Peter, the first pope, no pope has ever adopted the name Peter II. It is considered unlikely that any future pope would choose the name. Many popes have, however, had the saint's name as their baptismal name, most recently Benedict XIII (Pietro Orsini).
The likelihood of such a choice is further diminished by the final passage of the Prophecy of the Popes attributed to St. Malachy, a text which is well known in Catholic circles, though having no official standing. The Prophecy refers to a Petrus Romanus (Peter the Roman) as the final pope: he would "feed his sheep in many tribulations" and in the days of the "final persecution." This text invests the name of Peter II with an apocalyptic connotation.
The name Peter II has been adopted by several modern-day antipopes, particularly those whose claim to the papacy derives from a claimed personal divine revelation.
In the Left Behind series, Peter Mathews, a corrupt Catholic bishop, is elected pope and takes the name Peter II, subsequently becoming a leader of the Enigma Babylon One World Faith. This is a deliberate inversion of the passage from Prophecy of the Popes.
In The Third Secret by Steve Berry, an ambitious Vatican Secretary of State, Cardinal Alberto Valendrea, is elected pope and chooses the name Peter II.
The novel The Accidental Pope by Raymond Flynn and Robin Moore (2000) has a widowed American fisherman with four children elected pope by a fluke. He chooses the name Peter II in honor of Pope Peter I, who he describes as also having been a fisherman who had been married.
The Fifth Seal: The Rise of Petrus Romanus by Dennis R. Berry portrays Cardinal Vitale Milani, an evil and ambitious man who stops at nothing to assume the papacy, including mass murder. He chooses Peter II as his papal name.