Rex

Rex may refer to:
 Rex (title) (Latin: king, ruler, monarch), a royal title
 King of Rome (Latin: Rex Romae), chief magistrate of the Roman Kingdom

 Animals 
Dogs
 Rex (dog), once owned by Ronald Reagan
 Rex (search and rescue dog), a dog that received a 1945 Dickin Medal for bravery
Other animals
 -rex, a taxonomic suffix used to describe certain large animals