https://www.youtube.com/watch?v=K5qoJ6aRMmA Gaijin Goomba has a folklore history video on that featuring Nezuko. While depicted as big strong hulk smash type lately folklore has Oni as powerful spellcasters as well. He has another video about the enemies in Demonslayer where he comments that Oni used to refer to all youkai. Meaning the hidden ones.