Fandoms
So…this one is kind of complicated.
On one hand, The Birth of a Nation didn't create a fandom in the typical sense of the word. On the other, the film directly inspired the rebirth of the Ku Klux Klan, and though that can be uncomfortable to talk about, we think it's important to consider.
The Klan hadn't existed in earnest since the 1870's. Although they incited a great deal of violence during the Reconstruction period, the federal government put its foot down and squashed the organization. Plenty of other white supremacist hate groups popped up over the years, of course, but the Klan remained dormant.
It was dormant, that is, until The Birth of a Nation was released in 1915. The "second" KKK was formed that same year by a man named William Joseph Simmons in Stone Mountain, Georgia, where it remained a local organization until its expansion in 1921.
Interestingly, this new KKK had more in common with the Klan as depicted in the film than it did with the Klan as it actually existed. Those white costumes? From the movie. The burning crosses? From the movie.
This shows us just how important The Birth of a Nation was to this new class of bigots. In fact, it actually does make them seem like the type of fandom we see at Comic Con each year. They were racist cosplayers.
Of course, Trekkies and Bronies don't typically engage in terroristic campaigns of violent oppression. That's a big difference.