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

Hoaxes

A magazine page scan depicting Korechū

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] [2] [3] [4]

Aesthetic changes

Bulbagarden forums

Bulbapedia

Serebii.net

Pokemopolis

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.
Pokemopolis posted a cease and desist letter from a law firm representing Nintendo that claimed the site had tarnished Nintendo's image.
Pokedex.jp also changed its appearance to focus on Digimon.
TPM faked a database error on their forums.

Haruka Fanclub

Dogasu's Backpack

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

Real announcements misinterpreted as hoaxes included:

(unconfirmed) Chance (Japanese: チャーンス Chānsu), believed to be a pre-evolutionary form of Chansey is revealed as a new TCG card. (Pokébeach: [5] [6])