It could be a mosquito fish. They're similar to guppies. They also give them to people for free. I don't think it's a guppy. https://en.m.wikipedia.org/wiki/Mosquitofish

I caught some in a river and kept them. I thought they were guppies for years.

That's a juvenile female.

Also, they tend to be aggressive. They're also cannibalistic. They're also NOT in Poeciliidae, but they do give live birth. They're close though. They will kill fish from Poeciliidae, potentially with their penis (I'll let you guys search that). Also they give birth like crazy.

They ate ANYTHING I put in the tank. Bottom feeders were fine.

Looks like a guppy.

I agree with the other commenters. It looks like a juvenile guppy. I used to buy what was called "feeder guppies" for my painted turtle and they looked exactly like that. My turtle is too old and slow to catch them anymore so I now have a school of guppies cohabiting with him. After a couple of generations they start to develop more vibrant colors. Iridescent red and blue stripes and different patterns of dark dots. They look really pretty. They aren't really picky eaters. Mine nibble at the turtle pellets and the school had been thing for about five years on just that. If you do want to keep a school going make sure there are lots of places for the fry to hide, they are live bearers and guppies are notorious for eating their own fry.

Looks like a livebearer, can't really say further than that but I'm going with the guppy guessers.

Cool that you have some extra pets around, I can't identify them but regular fish feed should do the trick.