This freak show was pitched by Willis O'Brien, who did the special effects for the original King Kong. In O'Brien's script, a relative of the famous Dr. Frankenstein is hiding out in the jungles of Africa, but instead of stitching together pieces of dead humans like a normal Frankenstein, he opts to do his thing with parts of various giant animals for his own wacky experiments. Predictably, things don't go as planned, and Frankenstein is killed by his unnecessarily large creation. Word gets back to the U.S., where showbiz promoters plan to capture the creature and pair it with King Kong for a big stadium display show, because entertainment was very different before cable.
Both monsters escape and tear through San Francisco, eventually beating each other senseless atop the Golden Gate Bridge. RKO Pictures fully intended to make the movie, but sadly, they were unable to afford the special effects. They sought funding from Godzilla owners Toho, which bought the script and swapped in their own giant monster, saving us all from the pedants who would forever point out that the title should actually be King Kong vs. Frankenstein's Monster.