April 1, 2005 was an April Fool's Day that fell on a Friday.

Hoaxes

A magazine page scan depicting Korechū

Pokemopolis

Pokémon-related hoaxes this year included:

A new Pokémon, Korechū (Japanese: これュウ and これちゆぅ, sic) was revealed in a magazine page scan. PokéLord later admitted it had composed it as a joke. ^[1]
Chance (Japanese: チャーンス Chānsu), believed to be a pre-evolutionary form of Chansey, is revealed as a new TCG card. ^{[2] [3]}
Pokemopolis posted a cease and desist letter from a law firm representing Nintendo that claimed the site had tarnished Nintendo's image.
PokéMasters forums faked a database error.
Serebii.net forums had an elaborate custom avatar prank which angered many.
Legendary Pokémon showed a fake magazine page scan, revealing the shadow of a Pokémon named エイプリルフール — a katakana rendition of April Fool.

A Chance TCG card

PokéMasters forums

A scan showing April Fool

Aesthetic changes

Bulbagarden forums

Bulbapedia

A fairly predictable joke that recurs each April Fool's Day is an aesthetic change:

Bulbagarden forums forced all users to adopt a skin for a day, parodying the scheme of Serebii.net forums.
Bulbapedia was taken over by a cabal, and various texts were changed to reflect this.
Serebii.net used the classic Digimon-for-Pokémon substitution.
Pokedex.jp also changed its appearance to focus on Digimon.

Serebii.net

Haruka Fanclub

A variation on the previous theme, some sites change their focus:

Dogasu's Backpack

Real announcements misinterpreted as hoaxes included:

Pokémon Casewraps^[4] - a customise your consoles with Pokémon images, but as the information was released on April 1, there was confusion as to whether it was real.