E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
The horses of Castor and Pollux. Cyllaros and Harpagos. Seneca and Claudian give Cyllaros to Castor, but Virgil (Georgic iii.) to Pollux. The two brothers mount it alternately on their return from the infernal regions. Harpagos, the horse from Harpagium in Phrygia, was common to both brothers.