Do to really believe a snake talked?!

[John V]

That would be the equivocation fallacy actually (unless the tautological fallacy is another name for the equivocation fallacy? Personally I've never heard of it). Because that would be equivocating "existence" and "universe". It would be the equivocation fallacy but in this case it actually isn't because I'm not suggesting that the universe and existence are the same thing. I'm saying that "existence" has always existed, because that's a tautology. So whether the "universe" had a beginning or not, something has always existed. And so why would existence, which has tautologically always existed, require God?

Yes, it's actually equivocation followed by tautology. Tautology is another name for circular reasoning. It's something that's true by definition. As it's true, it's not strictly fallacious. It's considered a fallacy because it adds nothing to the argument, like saying "I know he's not married because he's a bachelor." There's also some misuse of converse or something back there where you were also throwing nonexistence into it.