A list of Pokémon who debuted in Pokémon Ruby and Sapphire, and their relatives. Introducing 135 Pokémon, of which only 2 are related to older Generations, this generation was the second largest until Generation V came around. 7 Pokémon related to this generation were introduced in Generation IV, bringing the grand total to 140. Seeing as in the first games of this generation, most old Pokémon were absent, and so several themes such as Fossils were brought back. This generation was also the first to diverge from the trio-uber-event dynamic of legendaries (although Lugia and Ho-Oh were a borderline break), by making the version mascots another trio, and introducing a pair of legends with no equivalent in older generations.
With the better hardware of the Game Boy Advance came better graphics, and the designs for Pokémon going forward started to get more complex and detailed. Many Pokémon were also created to showcase new features, ranging from Abilities and having their battling revolve around them, evolving using unique methods or simply existing for flavor (there isn't a real reason why Spinda needs a unique spot pattern gimmick, its just there to show off that it's possible). Many Pokémon were also designed to showcase double battles, and thus were designed with a closely related counterpart in mind: the most prominent examples including Plusle and Minun with their abilities of Plus and Minus that boost one another's powers when in Double Battles together, and Lunatone and Solrock being the signature Mons of the double-battling Gym Leaders Tate and Liza. It was also the first Generation to introduce the concept of Pokémon taking different forms, with Castform and Deoxys being the premiere examples.
For other generations, go here.