A hat. Castor is the Latin for a beaver, and beaver means a hat
made of the beaver's skin.
Took his new castor from his head.
Castor and Pollux
What we call comazants.
Electric flames sometimes seen in
stormy weather playing about the masts of ships. If only one flame
showed itself, the Romans called it Helen,
and said that it
portended that the worst of the storm was yet to come; but two or more luminous
flames they called Castor and Pollux,
and said that they boded
the termination of the storm.
But when the sons of Leda shed
Their star-lamps on our vessel's head,
The storm-winds cease, the troubled spray
Falls from the rocks, clouds flee away,
And on the bosom of the deep
In peace the angry billows sleep. E. C. B.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894