The term 'broken-down horse' can be broken down into four crossword solutions: NAG (3 letters), JADE (4 letters), CROCK (5 letters), and ROSINANTE (9 letters). NAG is a slang term for an old or worn-out horse, while JADE is a verb meaning to tire out or wear out. CROCK is an archaic term for a decrepit or worn-out horse, and ROSINANTE is the name of Don Quixote's horse in the novel of the same name. Together, these words describe a horse that is old, worn-out, and decrepit.