These are all the different groupings of Pokemon. A group is a collection of Pokemon that have a common feature.
Starter: The first Pokemon received at the start of their respective games along with their evolution line.
Fossil: The Pokemon which are revived from fossils in game.
Legendary: The rarest Pokemon in the games, as they are usually available once in the game. They are also meant to be the strongest Pokemon in the region. The mascot of each game is, with few exceptions, usually one of these Pokemon. They also have a legend of some sort connected to them.