Then it was a form of King Boo. It may have been distorted, but at least some of King Boo's traits gave birth to that form. I do not count him as officially King Boo, like the one from Luigi's Mansion, but he serves the same role.
Same with the King Boo in Mario 64. Stylistic choices.
I have to admit, I've always counted him the same as the other King Boo. I mean sure, the Japanese name is different, but it could have just been a bit of a mistake or theme (every boss is called [boss][species]).
As for King Boo in Mario 64... the translators just messed up there. He was called King Boo in Japan, then named 'Big Boo' in English, despite clearly being different to the existing Big Boo in the game.
Wario fan, Mario game modder and interested in every Mario RPG you can imagine!
I also run Wario Forums, which is a place to discuss the Wario Land and WarioWare franchises.