Castor comes from Greek mythology as the name of one of the twin Dioscuri and is also the name of a bright star.
Castor is a name from the oldest stratum of Greek mythology, one half of the Dioscuri — the Divine Twins, Castor and Pollux, sons of Leda and products of a night when both a mortal king and Zeus shared her bed. Castor was the mortal twin, famed across the ancient world as a tamer of horses and a master of swordsmanship, while his brother Pollux was the immortal boxer. The bond between them was so absolute that when Castor died, Pollux refused immortality alone, and Zeus transformed them both into the constellation Gemini — the Twins — so they could share alternating days between Olympus and the underworld.
The name itself likely derives from the Greek kastōr, meaning 'beaver,' though the animal's industriousness and its association with water may have carried symbolic meaning in the ancient Greek world. The Dioscuri were venerated throughout the ancient Mediterranean as protectors of sailors — a divine fire called St. Elmo's fire was believed to be their presence in storms — and as patron heroes of Sparta.
Their names appear in Roman prayers, in the Forum (the Temple of Castor and Pollux still stands, half-ruined, in Rome), and in early Christian writings where they became figures for brotherly devotion. Castor oil, the thick yellowish fluid pressed from the castor bean, takes its name from the same root, historically used as a medicinal and ritual substance. As a given name, Castor fell largely out of fashion through the medieval period, overshadowed by saints' names.
Its revival in the 21st century comes partly through the general appetite for Greco-Roman mythological names (Apollo, Atlas, Orion, Cassius) and partly through its association with the Gemini constellation, which holds perennial appeal. It is a name that rewards curiosity — every bearer eventually discovers the twins, the stars, the ancient sailors who prayed to them in storms.