The Spanish word 'yegua' (meaning 'mare' or female horse) comes from the Latin word 'equa', which also meant 'mare'. 'Equa' was the feminine form of the Latin word 'equus' meaning 'horse'. During the evolution from Latin to Spanish, the initial 'e' sound changed to 'ye' (a common sound change in Spanish), and the 'qu' sound evolved into 'gu', giving us the modern Spanish word 'yegua'.

While 'yegua' itself is a relatively basic Spanish word for 'mare', you might encounter related words like 'equino' (equine) in more formal or technical contexts, which maintains a closer spelling to the original Latin root.

English speakers can remember 'yegua' by connecting it to several English words that come from the same Latin root 'equus'. These include 'equine' (relating to horses), 'equestrian' (relating to horseback riding), and 'equitation' (the art of horse riding). While these English words kept the spelling more similar to the original Latin, they share the same ancient ancestor as the Spanish 'yegua'.