According to the Overlay Guide 2, "Evilswarm Castor" is not a corrupted "Constellar Pollux", but his twin brother who was corrupted.
Castor (Greek for "beaver" or "he who excels") is the twin brother of Pollux (also known as Polydeuces) in classical mythology. Castor is often portrayed as the mortal half-brother of Pollux, who is the divine son of Zeus.
This card's ATK is 50 points greater than "Pollux", and its DEF is 50 less.
This monster's spikes are on his left side, while those of "Pollux" are on the right.
This monster is right-handed, while "Pollux" appears to be left-handed.
This card's effect is similar to "Constellar Pollux", but focusing on "lswarm" instead of "Constellar".
Castor, following the "Constellar" naming scheme, is also based on one of the brightest stars in the Gemini constellation. Although Castor has the Bayer designation "Alpha," it is actually the second brightest in the constellation after Pollux.