The Spanish word 'mula' comes directly from the Latin word 'mula', which also meant 'female mule'. A mule is the offspring of a female horse and a male donkey. This is one of those cases where the word has remained remarkably stable both in form and meaning from Latin to modern Spanish.

Some related Spanish words include 'mulo' (male mule), 'mulero' (mule driver), and 'mulada' (a group of mules). These words are all derived from the same Latin root and form a family of words related to mules in Spanish.

The English word 'mule' is also derived from Latin 'mula', though it came through Old French 'mul'. This makes 'mule' and Spanish 'mula' cognates - words that share the same ancestral origin. The similarity in both sound and meaning makes this connection easy to remember for English speakers learning Spanish.