What Could Have Been: There were actually plans in the 90s to create an animated TV series with Disney of all things. While this never went anywhere a lot of the concepts from the pitch wound up mutating into what eventually became Gargoyles.
Dawson Casting: A weird case, as Hellboy is technically older than Ron Perlman, who plays him. Except that Hellboy is supposed to be Older Than He Looks and in his 30's appearance wise. Ron Perlman was in his 50's when he played the character.
If you were wondering why Luke Goss (Nuada) looks and sounds so familiar in heavy makeup, it's because he's played a supernatural patricidal prince before.
Meta Casting / Ink-Suit Actor: When discussing the film, Mignola and del Toro were considering who should play Hellboy. They each said they had a particular actor in mind and decided to each write their choice down on a napkin and turn them both over at the same time. They had both written "Ron Perlman."
What Could Have Been: Charlie Day was actually del Toro's first choice to play Nuada. However, Nuada's design looked too extreme on him so Luke Goss was cast instead. Day would later work with del Toro in Pacific Rim.