I refuse to believe that any Pokémon introduced after the first generation are canon. More specifically, any Pokémon with a Pokédex number over 150 (or under 1) in the first generation's Pokédex
does not canonically exist.
I know what you're thinking, "Oh, but Lala, What about Missingno or
Mew?!" It seems somewhat obvious that very few people consider Missingno a Pokémon. But, (if only to dispel any counter-argument) Missingno is a glitch and does not even have a Pokédex entry. In addition, Nintendo has attempted to erase its existence. More importantly, Mew does raise an interesting point. However, if you cannot recall from the various 'Pokémon' movies (which seem to [cleverly] contradict themselves) Mew went extinct long ago; ergo, he might have once existed but no more, and is thus un-canon.
Ok, so I've defined the first generation (the canon Pokédex). But why aren't any of the other generations canon, do you ask? I currently have a sound proof for all generations after the second. Starting in the third generation, there are
certified trainers who participate in battles
with more than one Pokémon per trainer at once. This fact would disgust any real Pokémon fan. As it is only obvious that the Pokémon League
strictly prohibits a trainer using more than one Pokémon at a time during a battle(c.f. Pokémon Anime Ep. 2.) This blatent contradiction to public Pokémon dogma is appalling and obviously proves that these games should not be considered canon. This further leads me to wonder what it is that Nintendo holds sacred.
It's definitely not canon rules.
I'll find a way to prove Generation Two un-canon, just you wait.