A similar event occurred during 2009 on /co/ with the mash-up initiative, but it didn't have anywhere near this level of output.
On November 20th, 2013, a thread was created on /co/ asking people to choose between Frankie (Foster's Home For Imaginary Friends) and Wendy (Gravity Falls) and was largely unremarkable until an anon proposed that they be combined and it just so happened that a wandering drawfag decided to answer that request. And so the fusion of Wendy and Frankie, Wanky was born.
But it didn't end there.
Red FridayEditThe following day, Thursday, a thread was started using the Wanky fusion and it was determined that Vicky (Fairly Odd Parents) should also be fused into this. The drawfag who answered the request previously returned (now using the name PlagueOfGripes) and drew the fusions of Wendy/Vicky and Frankie/Vicky; by then it was Friday. It was then deemed necessary that the fusions be fused, and once again the request was answered. The resulting character was named "Red Friday" as it was still Friday, all the fused characters were redheads, and the drawfag had deemed that the thread had been a bloodbath.
Then something even more remarkable happened, the Red Friday fusion received fanart, GOOD fanart, and a decent amount of it too. This prompted similar fusion threads to be made and those quickly expanded beyond /co/ where they were met with much success on /v/. It then spread to other sites and became a more mainstream meme for a time.
Some fusions have been develop as characters as characters on their own. Characters like that with their own pages include: