Rex, also known as Rex the Wonder Horse and King of the Wild Horses, was a 16 hands (64 in; 1.63 m) Morgan stallion who starred in films and film serials in the 1920s and 30s. His trainer was Jack "Swede" Lindell, who found him in a boy's school in Flagstaff, Arizona. He found that Rex had the unusual behaviour of trying to bite a whip when it was cracked. Wikipedia
The definitive Wikipedia entry for Rex. Wikipedia is the biggest multilingual free-content encyclopedia on the Internet. en.wikipedia.org/wiki/Rex_%28horse%29