Yes. A very common one is Missingo. Here is a walkthrough that has secrets at the bottom. Just remember that this walkthrough was made in 1999, so I wouldn't bother e mailing the guy who made it since he probably won't even remember making it. Here is the website:
http://www.gamefaqs.com/portable/gameboy/file/367023/5759
He is secretly green.
Gyarados doesn't any further on Pokemon FireRed.
Not any hoenn Pokemon but firered and leafgreen can catch alot of kanto and johto Pokemon ruby and sapphire can and can't.
you can't catch any hoenn Pokemon in firered or leafgreen you must trade hoenn Pokemon from ruby or sapphire or emerald to leafgreen or firered.
NO Yes there are!!! There's Zubat!!!
You can't catch Darkrai in Pokemon FireRed, it is a 4th generation Pokemon so therefore unavailable in any of the GBA Pokemon games.
Its impossible to capture any hoenn Pokemon in firered so you have to trade hoenn Pokemon from ruby, sapphire and emerald to get them.
Its impossible to catch any hoenn Pokemon in firered if you want hoenn Pokemon like groudon and kyogre trade them from ruby, sapphire or emerald to receive them in firered.
Anywhere, but they are rare.
yes
Nothing Happens
yes