In Action Comics #19 (October 1939), Zatara visits a city full of gorillas with human slaves, a full generation before Planet of the Apes. Crazily enough, this is not the fabled African Gorilla City that bothers the Flash decades later but a different city of gorillas in Mexico (?!?) made by a mad scientist who transplanted human brains into gorillas years earlier.
I found it horrifying that Zatara leads the humans to escape their bondage and kill their captors. I mean, you actually see blood and the decapitated head of a gorilla. Maybe Fred had a bad zoo experience as a child!
Love the human slave rockin’ that Z shield though!