The Spanish word 'pez' (meaning 'fish') comes from the Old Spanish word 'peçe', which in turn derives from the Latin word 'piscis', also meaning 'fish'. The evolution from Latin to modern Spanish shows a typical pattern of phonetic simplification, where the Latin 'sc' sound combination evolved to 'ç' in Old Spanish and finally simplified to 'z' in modern Spanish.

Several common Spanish words are related to 'pez', including: 'pescado' (which specifically refers to fish as food), 'pescar' (to fish), and 'pescador' (fisherman). While 'pez' refers to a living fish, these related words maintain the 'pesc-' root from the original Latin 'piscis'.

English speakers might recognize the connection to words like 'piscine' (relating to fish), 'pisces' (the astrological sign of the fish), and 'piscatorial' (relating to fishermen or fishing). These English words preserve the original Latin 'pisc-' root more closely than the Spanish 'pez'. The word 'fish' itself, however, comes from a different Germanic root and is not related to the Latin 'piscis'.