Can someone tell me where these characters came from?

[Pyro as Bill]

Bowser Junior: Who is he? He isn't a Koopa Kid. Has he ever been explained? Was he adopted? Was he Baby Bowser at some point?

Dry Bowser: Mega Bowser and GigaBowser I know about, I know about dry bones too but where did Dry Bowser come from?

Wario and Waliugi: Did Nintendo just make these two to personify Sega lovers? Fat, overweight, fart-joke loving Mario haters? Are they the original hardcore twins?

Baby Mario and Luigi: Are babalities a sign of desperation in the gaming industry? Mortal Kombat was getting stale so probably needed them but Mario.......?

 

If anyone can help me out I'll explain the secret truth about Birdo.