I think it's obvious that Hellboy is The Elephant Man, aka Joseph Merrick, in disguise. Most superheroes have secret identities, so why should Hellboy be any different? Just like it's impossible to fathom how nobody recognized Bruce Wayne just because he put on a mask, or why Lois Lane never said "Golly, gee, Clark.. what happened to your glasses?" when Superman was prancing around in his tights, it is beyond my understanding that nobody seems to see Joseph Merrick under all that red paint. I realize that many are skeptical, and so I have prepared a detailed visual analysis for their benefit:
Obviously the photo on the left is very old and the photo on the right is just an action figure, but I think the proof is strong enough to withstand such petty discrepancies.
1.) First and foremost you will notice that both Joseph Merrick AND Hellboy have freakishly enlarged right forearms and hands.
2.) Notice the penchant for shirtlessness shown by both subjects. This may seem trivial, but many a double-lifer has given himself away because he couldn't let go of those things which are so close to his alter ego's heart.
3.) How many people, real or fictious, have you seen around with two giant knobs protruding from their foreheads? My guess is just two: Joseph Merrick and Hellboy.
*flourish*
I really don't think I need say more.