according to Virgil, was the celebrated horse of Pollux (Geor., iii. 90), but, according to Ovid, it was Castor's steed (Met., xii. 408).
He, O Castor, was a courser worthy thee ... Coal-black his colour, but like jet it shone: His legs and flowing tail were white alone.
Dryden: Ovid's Metamorphose, xii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894