Venom is one of the most popular anti-heroes in the Marvel Universe. What began as simple costume change for Spider-Man turned into a monstrous addition to Spider-Man's rogues gallery, which then became an all-new lethal protector in Marvel Comics. This has been due to years of dynamic character development and new directions for the Venom character on the part of the publisher, but none of it would have been possible without the idea from a single Marvel fan.
Before Eddie Brock bonded with the Venom symbiote, the alien substance was in the possession of Spider-Man. Peter Parker came across the symbiote during Marvel's Secret Wars event, not knowing its dangerous intentions, simply seeing it as a new organic suit that augmented his strength and abilities, giving him an all new-look with a black and white suit. However, its darker true nature was soon revealed, and it began to exert its dominance over Peter, making him more violent and aggressive. However, Spider-Man resisted and tore the suit away from his body. Later, the symbiote would find its way to Eddie Brock, and the birth of the villain/eventual anti-hero Venom was born.
However, this entire series of events might not have happened if not for Marvel fan Randy Schueller. Schueller had entered a contest put on by Marvel, which was seeking amateur writers and artists to provide them with any ideas and/or designs that they would then potentially utilize in the future. Schueller was the one who came up with the black suit redesign for Spider-Man's suit, which Marvel's then editor-in-chief Jim Shooter purchased from Schueller for $220. Thus, Venom's origins began.
Schueller's original idea involved the new suit being made from same fabrication method used in the making of the Fantastic Four's suits which contained unstable molecules. The suit would also give him increased durability and stealth, though Marvel's writers went in their own direction with the symbiote concept. When the symbiote bonded with Eddie Brock, Venom became a full-fledged villain and enemy of Spider-Man, a concept and new character created by David Michelinie and Todd McFarlane, who wanted to revert Spider-Man back to his classic red suit.
Venom would go on to become the anti-hero he's now known as in the present, and even recently is being considered for an Avengers status, being personally vouched for by Captain America himself. At the moment, Venom is trapped in an alternate future with his son Dylan, joining that world's resistance movement against Codex, who has an entire hive mind army of symbiotes at his command. Over time, Venom has become just as popular as Spider-Man, the hero he once hated who has now become his ally, even getting his own film franchise starring Tom Hardy as the titular character. It's crazy to think that the staple of Marvel Comics that Venom has become all started with a 22-year old fan from Illinois who wanted to see Spidey get some new duds.