Antipope Christopher

Christopher (died after 904) claimed the papacy from October 903 to January 904. Although he was listed as a legitimate pope in most modern lists of popes until the first half of the 20th century, the apparently uncanonical method by which he obtained the papacy led to his being removed from the quasi-official roster of popes, the Annuario Pontificio. As such, he is now considered an antipope.

Source: Wikipedia — https://en.wikipedia.org/wiki/Antipope_Christopher