Funny villains that I like. (Fassad and Nightmare are evil and need to perish in the fires of mount doom because they're extremely annoying and evil and UAAHA)
They were also both the final bosses of the second game in the series but okay.
They are both gargantuan robots (Well technically Nightmare is a cyborg) that are very difficult, and created by the protagonists of the game as mass weaponry but then corrupted and were forced to kill Samus.