What even is a “normal dragon” in the world of OP lmfao. These things are so rare that Ryuma slaying a “normal dragon” would absolutely be a historical feat, how many dragons have we ever even heard of in the OP world that weren’t made by devil fruits, or weren’t the result of scientific meddling. These things are mythically rare, so yes Ryuma slaying some random inconsequential Wano dragon is absolutely 100% possible and 100% probable.
This is just like when people thought Akainu scarred Kaido lmfao, 100% improbable from the start. As soon as y’all realize Oda’s world is a bunch of completely disconnected worlds and histories plagiarized from irl mythologies, the sooner you see through his illusions. Oda will not cross Nordic and Japanese mythologies, name literally 1 other time in the history of the manga this has happened lol.