Mules are a hybrid animal, created by crossbreeding a male donkey and a female horse. The term 'mules' can also refer to the offspring of other similar crossbreeds, such as a male horse and a female donkey. The three crossword solutions for the term 'mules' are 'hardheads' (9 letters), 'ASSES' (5 letters), and 'SLIPPERS' (8 letters). Hardheads is a slang term for mules, referring to their stubbornness and unwillingness to cooperate. ASSES is another slang term for mules, referring to their donkey parentage. SLIPPERS is a term used to describe the shape of the mule's hooves, which are narrower than those of horses.