Rex in poster for The Law of the Wild
|Sire||Pride of Mountain Vale 6986|
|Dam||Black Bess 0218|
|Maternal grandsire||The Admiral 4871|
|Foaled||1916 or 1917|
|Trainer||Jack "Swede" Lindell|
His trainer was Jack "Swede" Lindell, who found him in a boys' school in Golden, Colorado. He found that Rex had the unusual behaviour of trying to bite a whip when it was cracked. Lindell encouraged this and would often stand behind the camera to get a dramatic shot on film. Lindell never left Rex alone on set unless he was locked in his own trailer.
During filming of The Law of the Wild Rex made a commotion on set. When he charged the camera (with Lindell behind it) as intended he did not stop when Lindell gave the signal to do so (by holding his whip in both hands). He reared, knocking over several reflectors and causing the cast and crew to scatter for cover. Rex chased one actor, Ernie Adams, who attempted to hide under a car. Rex dropped to his knees and attempted to bite Adams with his head thrust sideways underneath the car. Lindell managed to call Rex off by simply cracking the whip, after which the horse calmly walked over to him. When William Witney, working as an assistant director on the serial, made Stranger at My Door (1956 film) he described the event to trainer Glen Randall and the scene was recreated for that movie.
In one scene from No Man's Law, Rex saves the modesty of a young woman (Barbara Kent) swimming in the nude from a pair of rowdy villains. Chasing one around in circles, rearing up and bucking like a wild mustang, until he finally runs him off of a cliff, he sneaks up behind the other and nudges him with his nose over the ledge and into the watering hole. He then nose prods the now-clothed young woman back to her father.
- Black Cyclone (1925)
- The Devil Horse (1926)
- Wild Beauty (1927)
- No Man's Land (1927)
- No Man's Law (1927)
- The Harvest of Hate (1929)
- Hoofbeats of Vengeance (1929)
- The Law of the Wild (1934)
- The King of the Wild Horses (1934)
- Stormy Actor (1935)
- Black Stallion or King of the Sierras (1938)