Theory: Sunshine King Boo's Identity

Abstract
The King Boo seen in Super Mario Sunshine is not the real King Boo, but rather a different King Boo created by the Paint/Goop/Goo from the Magic Paintbrush.

Arguments

 * Argument 1: The “King Boo” in Super Mario Sunshine has a completely different design to the King Boo that appears in the Luigi’s Mansion series, Super Mario 64 DS and other various Mario spinoffs.


 * Argument 2: In Japanese, this character is referred to as Boss Boo rather than King Boo.


 * Argument 3: In Luigi’s Mansion, King Boo is trapped inside a Painting by Professor E. Gadd and Luigi. He is sealed in this painting until the painting is sold in a yard sale by the Professor and the real King Boo escapes. This means that King Boo was likely still trapped within the painting at the time when Super Mario Sunshine takes place, warranting Bowser to create a “new” King Boo in his absence.


 * Argument 4: The “King Boo” seen in Super Mario Sunshine explodes into Goop once defeated, just like other enemies created by the Magic Paintbrush.