The Spanish word 'rey' meaning 'king' comes directly from the Latin word 'rex' (also meaning 'king'). The transformation from 'rex' to 'rey' follows a common sound change pattern in the development from Latin to Spanish, where the final 'x' sound softened to a 'y' sound.

Some related Spanish words include 'reina' (queen), 'real' (royal), and 'reino' (kingdom). All these words share the same Latin root 'rex' and form part of the semantic field relating to monarchy and royalty.

The English word 'regal' comes from the same Latin root 'rex'. Other English words from this family include 'royal', 'reign', and even 'regulate' (which originally referred to ruling or controlling). The word 'rex' itself is sometimes used in English, particularly in scientific names and historical contexts.