Here's an interesting panel discussion topic from
Eville-Con cosplay horns. The most obvious ones are the troll horns from Homestuck. When we first started selling a lot at anime conventions, Homestuck was enjoying its first big surge of popularity, and it seemed like every other person at a convention would be cosplaying a Homestuck troll.
Because the creator of Homestuck has refused to license any products from his properties, all Homestuck troll horns are amateur work. At first there were some people creating and selling troll horns by hand in various conventions' artist alleys, which the conventions generally allowed as long as they were genuinely hand-made and each pair was unique. However, the creator of Homestuck subsequently requested that there be no sales of any form of fan art from his work, including horns and other costuming pieces as well as prints, paintings and other two-dimensional art.
However, while Homestuck troll horns may be the most obvious form of costume horns we see at anime conventions, they're by no means the only variety. Another popular type is the "cute little devil" with or without bells and other ornamentation, often seen in anime when an otherwise cute character is giving in to a naughty impulse, and cosplayers often wear them to suggest a character who lies the boundary of naughty and cute.
And of course you have the traditional Japanese troll or oni, particularly as reinterpreted through the anime Urusei Yatsura, sometimes translated "Those Noisy Aliens," since Lum and her family are invading aliens whose appearance plays upon the tradition of the oni.