In Pokémon Unite, there are a variety of Pokémon you can pick to play as. These all have different attacks, roles and stats and many have you start as the base evolution and evolve up to the focuse Pokémon while in play.
These Pokémon are obtained by purchasing the License to use them in the shop, or earning their license as a reward, but all can be played in the Practice Area regardless of if you have obtained them yet. You will also get several free Unite Licenses which rotate each week, butthese cannot be used in Ranked Battles.
You may also search for Pokémon by their stats using the chart below
List of all Playable Pokémon