Ignore the comments about endangered species. People pulling your leg is all that is. I dunno, I am torn. It's very obviously a killifish. As to which one, man, hard to call. It has some pretty significant fin damage and is extremely skinny. I worry that it is underfed. Anyway, look into Pachypanchax sakaramyi and Aplocheilus lineatus (gold phase or color morph). The latter is most likely. AKA a Golden Wonder killifish. Oh, and one more thing. They vary quite a bit in coloration so some look wonderful while this looks like... well, I am sure it is more colorful in perosn.