Rex

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Latin, literally ‘king’.

wiktionary

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From the Latin rēx(“king”), referring originally to rabbits of the Belgian "castorrex" breed, so named because their fur was similar to that of beavers. Entered English around 1920. Doublet of rajah and roy.

etymonline

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rex (n.)

"a king," 1610s, from Latin rex (genitive regis) "a king," related to regere "to keep straight, guide, lead, rule" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule;" source also of Sanskrit raj- "king;" Old Irish ri "king," genitive rig).