Have any antipopes other than Antipope Hippolytus of Rome ever been canonized as saints by the Roman Catholic Church?
Only one other antipope than Hippolytus of Rome is considered a saint in the Catholic Church.
The antipope Felix II has been given the status of a saint in the Roman Martyrology with a feast day of July 29.
According to the "Liber Pontificalis", which may be registering here a reliable tradition, Felix built a church on the via Aurelia. It is well known that on this road was buried a Roman martyr, Felix; hence it seems not improbable that apropos of both there arose a confusion (see FELIX I) through which the real story of the antipope was lost and he obtained in local Roman history the status of a saint and a confessor. As such he appears in the Roman Martyrology on 29 July. - Catholic Encyclopedia