So there's this page of TV Tropes that goes like this

>Rule 34 – Creator Reactions > Video Games > Pokémon
>A rare exception to the general attitude of Japanese companies to doujins was a doujin that involved Satoshi (Ash) turning into a werewolf and doing unspeakable things to Pikachu. Nintendo came down hard on that one. They also filed a cease and desist against Paheal, the largest Rule 34 based site on the net, upon discovering that the Pokémon porn were receiving more hits than the official website (as such, the Pokémon tag was changed to Porkyman to prevent further C&D cases).
>In general, The Pokémon Company is fine with Rule 34 as long as it doesn't involve the Pokémon themselves. After an artist released a doujinshi involving Hilda and Tepig, they forced him to discontinue it. As a result, the sequel he was writing had humans only. Some artists also seem to get around the restriction by only using incredibly humanoid mons, such as Gardevoir. Mon/mon also appears to be mostly considered less objectionable than mon/human.

And it's not that I have troubles to believe any of it; I have noticed how Pokémon doujins usually split between all-human or all-Pokémon, and the use of Pokémon iconography in the former is pretty tenuous.
But is there any source for this? Would love to know.