The Green Goblin is one of Spidey's oldest and deadliest foes. He first appeared in Amazing Spider-Man #14 (written by Stan Lee, illustrated by Steve Ditko) and has since continued to have an effect on Spider-Man and Peter Parker in ways that would push anyone else into quitting their superhero career. Among the worst things he's done, Norman Osborn has tricked Peter Parker into believing that his parents had returned, and seduced and impregnated Peter's girlfriend Gwen Stacy before killing her by throwing her off a bridge.
Recently, following a brief reign of terror as the Goblin King, Norman went into hiding only to resurface in the country of Symkaria, though he remained stripped of his Goblin abilities and persona. He and Countess Karkov staged a coup and took over the nation, transforming it into a giant weapons manufacturer, filled with goblin serum-infected soldiers. Thankfully, Spider-Man and Silver Sable were able to free the country and defeat Karkov and Norman, but not before the latter underwent a botched operation that left his face warped and more goblin-like.
Norman was defeated and disfigured, but he escaped. Afterward, he embarked on a journey to regain the Goblin within him through mysticism, a journey that evidently didn't end well as we saw in Amazing Spider-Man Vol 4 #32 (written by Dan Slott with art by Greg Smallwood). Here, Norman was rejected by the monks of the temple due to the fact that he's pretty much evil incarnate.
While the Green Goblin was thought to have been well and and truly gone, a new event was recently teased by Marvel, one set to introduce a new threat. The trailer revealed the return of the Green Goblin and one final confrontation between him and Spidey before giving us a look at something more terrifying: The Red Goblin. We don't know much about this new character other than what we can gather from the fantastic image. It does seem pretty certain that there's clearly a supernatural element to him; after all, he's got a devilish tail and hellish red smoke spilling from between his jaws.
There are a few theories surrounding this new menace and since this is the world of comics we're talking about, they're all equally plausible. He could be a new villain entirely. He could be a combination of the Goblin and the Carnage symbiote. (Just look at that mouth!) Or -- and this is the most likely -- he could be the result of a pact of some kind between a desperate Norman Osborn and Mephisto.