Bigfoot.
Like, there's probably not any alive today. Probably not any in the last thousands years even.
But at some point in human history, that shit existed. Might even just be stories about gorillas that ancient hominids took out of Africa and kept telling afterwards.
For a stone age human, a silverback might as well been a mythical animal. And they're deceptively quiet. Imagine just walking thru the forest and seeing a fucking Silverback standing 10 feet away staring at you.
That story would keep getting retold long after people forgot what a gorilla was.