Jirachi

The third-generation (Generation III) of the Pokémon franchise features 135 fictional creatures introduced in the 2002 Game Boy Advance games Pokémon Ruby and Sapphire.

The following list details the 135 Pokémon of Generation III in order of their National Pokédex—an in-game electronic encyclopedia that provides various information on Pokémon—number. The first Pokémon, Treecko, is number 252 and the last, Deoxys, is number 386. Alternate forms that result in type changes and Mega Evolutions are included for convenience.

Nintendo Life noted in a retrospective that the third generation of Pokémon has a very different "feel" from the two generations that came before it because almost all of its 135 new Pokémon – save for Azurill and Wynaut – have no relation to those of the previous generations. Unlike generation 1 and 2, two of the "starter" Pokémon of Ruby and Sapphire gained a secondary typing in their final form, giving them a wider range of abilities. Ruby and Sapphire features two "Mythical Pokémon" – Jirachi and Deoxys – both of which became available to coincide with their respective anime movies.

Pokémon Ruby and Sapphire significantly increased the amount of "Dark"- and "Steel"-type Pokémon in the series, as only a few Pokémon in previous generations used these typings. Hardcore Gamer also noted that many of the new Pokémon made use of "dual typing", where Pokémon have both a primary and a secondary type; this wasn't nearly as common in Red and Blue or Gold and Silver.

A new mechanic introduced in X and Y called Mega Evolution—a temporary form change akin to normal evolution—was added for more dynamic battles and stemmed from the concepts of bonds and evolution. A variant of Mega Evolution called "Primal Reversion" was introduced in Omega Ruby and Alpha Sapphire; this mechanic is exclusive to the legendary Pokémon Groudon and Kyogre.

...
Wikipedia