In general the term "mascot" is used to identify fictional characters (more rarely non-fictional people, animals or objects) used to represent or bring luck to a group of people, a company or such. To this Wiki specifically, though:
A Mascot is any fictional character that doesn't belong to any specific fictional series.
Since we only classify fictional series, not singular characters, informations about mascots are generally ignored in this Wiki.
This definition is used for most of the typical mascot characters: for example Benny the Bull is a fictional bull, the mascot of NBA team Chicago Bulls, but does not originate from any fictional series.
This definition however also includes characers that are not necessairly used as proper mascots: for example Miis (though their status as fictional characters is arguable) are the player's avatar in some Nintendo games, and while not considered official Nintendo mascots, they fall in our definition of mascot, since they don't originate from any specific fictional series (they are from the console's built-in software itself).
On the other hand the definition excludes characters that are used as mascots, but belong to a specific series: for example Sonic the Hedgehog is used as the mascot of Sega, but he originated from the Sonic series, so we don't consider him a mascot.
Note that the definition says that a character to be considered a mascot must not belong to a specific fictional series, independently from its creation: even if a character was created as a mascot it might start to belong to a specific series, therfore becoming an actual fictional series character, and no longer be considered a mascot. Many commercial mascots fall in this category, since we consider TV commercials actual fictional series (see Commercial rule): for example the Nesquik Bunny originated as the Nesquik mascot, and at first was only featured on cans of the product, but later received his own series of TV commercials and comic books (the Nesquik series).
As with fictional series, however, a mascot character must be copyrighted to be considered an actual fictional character, otherwise any character scribbled on a piece of paper would be considered, making a classification pointless and also impossible.
Given our definition of mascot, the rule that regulates their appearances in fictional series states:
If a mascot character appears in two or more series, it creates an undirect in-universe link between them. Every other appearance of a mascot, in any form, is not notable.
This is because while we don't need informations about singular characters, their appearances in two or more series creates a connection between their fictional universes and is therefore notable.
This means however that a mascot character appearing in only one fictional series is not a notable appearance: for example the Bullshit Man, a character originating from the Cinemassacre series You Know What's Bullshit (that's not considered fictional, since it's only a series of reviews), made two appearances in the Angry Video Game Nerd series, but it's not notable since this only connects AVGN to a mascot character, it doesn't connect it to another fictional series.
Also the appearance is only notable if it is the actual fictional character, not if it is a person in a suit: for example in the videogame NBA Jam Tournament Edition and some subsequent games in the series, many mascots appear as secret selectable players, but they clearly are just people in suits, so this is not considered a reference to the fictional character, but rather to the real life mascot, so it's not notable. In general, only in-universe links regarding mascots are notable; possible type 2 links (like a person in a suit) or type 3 links (like a character based on a mascot) are not considered notable at all, since they are not references to fictional series.