Rex is a masculine given name derived from the Latin word "rex", meaning "king". It originated in ancient Rome during the Roman Republic and Empire periods, and was used as a title for the ruler or sovereign of the Roman state.

The name Rex gained popularity during the Roman era, and was often associated with power, authority, and leadership. It was not uncommon for Roman rulers or members of the nobility to bear this name or use it as a cognomen (a personal surname or nickname).

One of the earliest and most notable historical references to the name Rex can be found in the writings of the Roman historian Livy, who mentioned a Roman general named Quintus Fabius Maximus Rullianus, also known as Rex, who lived in the 4th century BC. Rex was his cognomen, indicating his regal bearing or qualities.

In later centuries, the name Rex continued to be used, albeit less frequently. Some notable individuals with the name Rex throughout history include:

1. Rex Theodericus (454-526 AD), also known as Theodoric the Great, was the king of the Ostrogoths and ruled over a significant portion of the former Western Roman Empire.

2. Rex Horne (1828-1909) was an American politician and lawyer who served as the 13th Governor of North Carolina from 1891 to 1893.

3. Rex Ingram (1892-1950) was an American film director, producer, and screenwriter who directed several notable silent films, including "The Four Horsemen of the Apocalypse" (1921) and "The Prisoner of Zenda" (1922).

4. Rex Stout (1886-1975) was an American crime writer best known for his series of novels featuring the fictional detective Nero Wolfe.

5. Rex Harrison (1908-1990) was an English actor renowned for his performances in films like "Cleopatra" (1963), for which he won an Academy Award, and the stage and film adaptations of "My Fair Lady" (1956 and 1964, respectively).

While the name Rex has retained its association with royalty and leadership over the centuries, it has also taken on additional connotations related to strength, power, and masculinity.