Castor \c(a)-stor, cas-tor\ as a boy's name is of Greek origin, and the meaning of Castor is "beaver". In classical myth, along with Pollux, one of the heavenly twins immortalized in the constellation Gemini. They were considerd the patron gods of seafarers, appearing to them in Saint Elmo's fire.
Castor has 3 variant forms: Caster, Castorio and Kastor.
For more information, see also the related name Pollux.