(4 syl.) A wretched jade of a riding-horse. Don Quixote's horse was so called. (Spanish, rocin-ante, a hack before.)
“It is the only time he will sit behind the wretched Rosinante, and it would be Quixotic of him to expect speed.” —London Review.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894