A variant transcription of Russian Pyotr, and also the Polish and Belarusian form of Peter, classic and still very popular there.

St. JOhn is mainly (but rarely) used by Catholics in England -- there's one in Jane Eyre. This would probably be too cumbersome for most American kids.

Thanos originated as a short form of Athanasios, a Greek name meaning "immortal." It is composed of the elements a, a negative Greek prefix, and thanatos, meaning "death." St. Athanasius, who used the Latin spelling of the name, was a bishop of Alexandria and known for his opposition to Arianism.