Well you've got two choices, a eternal universe or a creator.
That assumes that time is meaningful for the universe as a whole - there are physical models where time exists only
within the universe. As for a 'creator', there are a number of physical models where physical processes in a larger 'metaverse' create universes.
There's no reason at all to suppose that cosmic serpents, oceans of chaos, warring demons, magical entities, paternalistic anthropomorphic deities, etc., had anything to do with it; they're clearly the products of vivid human imaginations.
As far as we know, everything has a beginning.
We don't really know that. As far as we know, our universe was once very hot and very dense. We don't know the prior history of that hot dense state. Everything we know subsequently condensed out of that hot dense stuff as it cooled; IOW, the things we know are simply different arrangements of that stuff. They only have a 'beginning' in as much as we identify certain arrangements of that stuff as particular things, and the arrangements of that stuff change over time.
But if
everything has a beginning, you'd necessarily have an infinite recursion of beginnings - the universe would have a beginning, whatever it came from would have a beginning, and so-on. If you want some kind of creator to be exempt from that, it would be a
special pleading fallacy. But if you insist, I will invoke Occam's razor and say that if
something is exempt from having a beginning, then it should be the universe itself, which we know exists, and we can then drop all redundant creator concepts for which there is no evidence or necessity.