Fanon

Fanon (short for fan canon) is comprised of rumors, lies, and/or statements made up by fans of a series that are not part of official canon. Creators of fanon material may or may not publicly acknowledge its absence from canon.

Some well-known fanon statements:
 * Xelloss was a human once: Possible, but unlikely, as nothing exists in any official Slayers material that would even remotely hint this, actually, there are hints which point to the opposite.
 * Xelloss' last name is Metallium and he is the general-priest of the Greater Beast: In the sixth novel, Xellos explicitly stated that his full name and title is Xelloss, the Priest. He does not use Zelas' last name, if that could be called a last name at all.
 * Zelas and Luna Inverse are friends: See the article of Greater Beast Zelas Metallium for details.
 * Deep Sea Dolphin is insane and was driven insane by Xelloss and Zelas: Again, such a fact was never mentioned in any official material. When Dolphin appears in the last novel, we are given the impression that she is actually quite sane.
 * Filia Ul Copt is the last golden dragon: After the events of Slayers Try, Filia is indeed the last golden dragon who served Firelord Vrabazard, but the servants of the other three shinzoku still remain.
 * Mazoku cannot love: Considering the mazoku's vulnerability to positive emotions, this might actually be true. What still makes it a fanon fact is that, contrary to popular belief, this was never stated by Hajime Kanzaka, and thus still can be a subject of debate.
 * Lei Magnus was the lover of the Lord of Nightmares: While not true per se, strangely enough this rumor is actually rooted in a real comment from the author. The Lord of Nightmares, when appearing beside Kanzaka and Lei Magnus in the afterwords of the novels, is actually copying the personality of Lei's long-dead human lover. Of course, this still does not mean that Lei was/is in love with the Golden Lord herself.
 * The Lord of Nightmares is a mazoku: See her article for details.
 * Zelgadiss Graywords is one-third mazoku: A misconception based on Zelgadiss being one-third blow demon.
 * Zelas and Dolphin hate each other: The two appear together in volume 15 of the Slayers novels and seem to get along quite fine with one another.
 * Valgarv is general/priest of Chaos Dragon Garv: Although Valgarv was very close to Garv, he is not one of Garv's priests or generals, as Garv already had a priest (Raltark) and a general (Rarshart). See Mazoku generals and priests for more details.

The above examples are usually derived from misconceptions made by English-speaking fans that had no access to the Slayers novels, and would therefore make assumptions based on what information they did have at the time. For example; fans were lacking the same info as the Japanese fans and they had no idea that Luna came across Dilgear, adopted him and called him Spot. If they did, the rumor of Luna being friends with Zelas would have never cropped up. Many other misconceptions come from fans taking fanfiction too seriously (again due to limited knowledge) or from poorly made dubs and fansubs.