Get a picture of it lolit looks like a tiny bristlenose pleco. Its maybe a 1/4 of an inch long. The only other fish it might be is a corydora fry but it just doesn't look like it.
If you have two bettas then i'd guess it could be betta fry?a betta, zebra dainos and neon tetras. It's actually my daughter's tank that which ends up with odds and ends from my other tanks.
Very unlikely, male and female bettas cannot remain around each other after spawning as the male will kill the female after spawning and I'm pretty sure the OP would have noticed a male tending a bubble nest for several weeks. Betta spawning is difficult to do when trying, hardly possibly by accident. Zebra fish however, spawn very easily and often. But they are egg scatterers, so its quite possible that the cory cleaned up the scattered eggs and missed all but one or a few.If you have two bettas then i'd guess it could be betta fry?