When this film was first conceived it was supposed to have been a follow-up to King Kong (1933), but was never made. However, an early B&W version of the "cowboys in Africa" footage was shot, and wound up being used in Mighty Joe Young (1949).
Special effects master Ray Harryhausen has said that sequence of the elephant performing its act and its subsequent fight with Gwangi were done with no shots of a real elephant because no such animal was available. However, there is an elephant in the very early scene of the Wild West show's parade through the town, and Ray's animation puppet is a perfect double for it.
The roping of Gwangi was achieved by having the actors hold on to ropes tied to a "Monster stick" that was in the back of a Jeep. The jeep and stick when filmed with Gwangi are on a back rear projection plate and hidden by his body and the portions of rope attached to his body are painted wires that are matched with the real ropes.
Special-effects pioneer Willis H. O'Brien began pre-production at the RKO-Pathe Studios on a story by Harold Lamb about a huge T-Rex called "Gwangi", with John Speaks as producer, in 1941. The project was canceled when studio management was changed.