>>213437344
There are many different words for monsters in Japanese, youkai, oni, mamono, kaibutsu, and so forth. In the case for oni, it comes from the same Chinese character that's the generic word for ghost. The kanji that's read on its own is pronounced oni, but is read as "ki" in compounds (hence kimetsu).
So you have a word like vampire in Japanese, which is 吸血鬼, kyuuketsuki, sucking blood oni (or ghost, or monster), but Japanese people don't mentally break down their compounds into individual parts anymore than you would if you heard doghouse, so it's just the word for vampire. Likewise in games like tag the one who's it is the "oni." So it's not accurate to say it's the word for monster, but rather can be a word for monster. That's why the "oni" in Demon Slayer are so different from the archetypical ones you'd find in the story of Momotaro and the like.