You're confused because it has always been confusing.
Sega owned the rights to the Wonder Boy trademark while Westone owned the copyrights to each game.
That's how we ended up with Wonder Boy, Monster Land, Monster World, Adventure Island, Dynastic Hero, Dragon's Curse, Bikkuriman World, Saiyūki World plus the Brazilian Monica reskins. They're all essentially the same thing.
I mean they released a game titled Wonder Boy V: Monster World III.
Yes. That's the title of a single game.
It's always been confusing.
Now to add to the confusion we have the recently released remake of Wonder Boy III The Dragon's Trap AND the newly created Monster Boy and the Cursed Kingdom.
I actually think it's kind of charming that the recent releases continue the tradition of confusion in the fragmented Wonder Boy/Monster World universe.