Playable species include a variety of Lythians, all of which are able to participate in any and all parts of the game.
Companion species are friends to playable species. They have fewer activities that they are permitted to participate in, though they may have some skills which are used to help their playable "owners" out in various ways.
Customizable Pet Species
Pets come in a large variety of species, but some are more common and varied than others. You can see all common pets via your bestiary, but customizable ones will be detailed here. Pets are considered collectibles that your Lythians can own, and do not participate in activities.