The so called 'mules' in 'Two Mules For Sister Sara' are
donkeys, or, in Spanish, 'burros' . A 'mule' is the offspring of a
female horse ('mare') and a male donkey ('jack'). A 'hinny' is the
offspring of a male horse ('stallion') and a female donkey
('jenny'). Most westerns filmed in the southwest feature numerous
donkeys/burros with mules being somewhat more of a rarity.