Ascended Fanon: Mamare has gone on record several times to state that, if he likes a particular bit of fanfiction done on the series, he'd be more than willing to add its details to official canon.
A notable example is the fanfic D.D.D. Diaries, which details the command structure of the eponymous guild, as well as its personalities, such as the female assassin in the skimpy outfit that appears to the right of Krusty during the assault on the Goblin General. Who also happens to be a G.I.R.L..
Hien comes from a fan fic. Most of Crescent Moon's members in the anime were taken from Honey Moon Logs, just like how several members of the West Wind Brigade were from their guild's spin-off manga.
Aside from the nine members who were trapped in the Apocalypse, most of the Debauchery Tea Party's members' names and descriptions came from a fan book.
An excellent example of this would be the players who joined Akatsuki◊ in the anime taking down the Akiba murderer. Anyone who wasn't Akatsuki, Raynesia, Rieze, Mikakage, or Tatara, had a manga spinoff of their own (Crescent Moon, West Wind Brigade) was either a fanfiction character (up to and including the missing D.D.D. Der Klauen member Kushi Yamata) or a transplant from another part of the franchise (Seine, who's a character in the Log Horizon REPLAY books, and Endou Akiba from the tabletop RPG).
Minori is sometimes called "The Villain With A Smile," because she's The Apprentice to "The Villain In Glasses".
Due to Jovan Jackson's (an African-American) acting re-imagining Nyanta from a upper-class gentleman to a literal cool cat, so to speak, some fans have taken to affectionately refer to the dubbed version as "Sexy Black Nyanta".
Scrub Horizon: The Cast Herd of the junior members of the Log Horizon guild (Tohya, Minori, Isuzu, and Rudy), along with Serara (who's part of the herd despite being a member of the Crescent Moon Alliance).
No Export for You: So far, only the West Wind Brigade spinoff has been slated for international release. Everything else, meaning the other spinoffs (Kanami's side story, Nyanta's Recipe) are unavailable to read for Western followers of the series.
Now Averted as the light novels have been licensed by Yen Press.
Similarly Named Works: Not an exact case of it, but a French MMORPG-centered franchise named Noob happens to take place in a game named Horizon.