From Homestar Runner Wiki
Onion Bubs is an onion version of Bubs that, according to Strong Bad, was one of the many temporary replacements for Original Bubs. Since Strong Bad apparently made up Original Bubs, it initially seemed as though the same was also true of Onion Bubs, but it has since made other appearances.
In terms of appearance, Onion Bubs is nothing more than an onion with Bubs's face drawn on it. Curiously, despite allegedly replacing Original Bubs, his eyes and mouth are shaped more like those found on regular Bubs.
Strong Bad describes him as a "fan favorite", and, indeed, his appearance was met, along with subsequent appearances, with great cheering and a single overenthusiastic voice shouting his name over and over again.
Onion Bubs speaks for the first time in Doomy Tales of the Macabre when Bubs's head flies off after being chomped off by his monster-transformed concession stand. As the head lands, Onion Bubs says "Welcome to my world!" and laughs evilly. Strong Sad was just making this up, so it's possible that Onion Bubs is not actually able to talk.
|Old-Timey Bubs | The Thnikkaman | Shark-Tooth Bubs | Baron Darin Diamonocle | Bubsamecium | Zubs or Coach B | King Bubsgonzola Supreme | Dry Ragamuffin | Original Bubs | Onion Bubs | Bubz|