Mkay, so I was wondering....Mewthree is a Pokemon. Only heres the problem: HE'S FAKE. So if you were to put it in a fanfic, how would this reflect on the fic? Would it make it look bad? Would it be amazing? Or does it depend on how the writer uses it in the story?
Many many Pokemon fanfiction writers (and, actually, fanfic writers of other canons that have little monsters) make up their own Pokemon. They're called "fakemon", because they
are fake, according to official canon.
The one thing is that with fakemon, the author can make it seem like the Pokemon they created fits into the canon world because, if one thinks about it, there probably
are more than just 493 Pokemon out there. I've seen fics where the fakemon just seem to belong to the Pokemon world because of how the author handles them. (For a great example of this, read "The Quest for the Legends" by Dragonfree. For a non-OT fic, there's also "The Origin of Storms" by Sike_Saner.)
So, pretty much, it all depends on how the writer uses the fakemon. If the fakemon does not seem to "fit" in with the rest of the canon Pokemon, then that aspect of the story really isn't that good. On the other hand, if the fakemon fits, then that aspect of the story
is good.