In addition to DanmakuGrazer's answer, there are some other named groups of pokémon to be aware of, although both have unofficial names:
This group is for pokémon who are usually encountered in ways similarly to regular legendaries (one per game, usually encountered in the overworld, story talks about their rarity) but they aren't nearly as powerful as other legendaries. This group mostly comprises pokémon who have a Base Stat Total (BST) of 600 or below, and can't be bred.
A key feature of this group is that all of its pokémon are allowed in places where other more powerful legendaries are banned, such as online PvP battles or in-game facilities like the Battle Tree. All pokémon in this group have a green background in the Gen 7 pokédex.
The pokémon in this group are: Articuno, Zapdos, Moltres, Raikou, Entei, Suicune, Regirock, Regice, Registeel, Latias, Latios, Uxie, Mesprit, Azelf, Heatran, Regigigas, Cresselia, Cobalion, Terrakion, Virizion,
Tornadus, Thundurus, Landorus, Type: Null, Silvally, Tapu Koko, Tapu Lele, Tapu Bulu, Tapu Fini, and all Ultra Beasts.
These pokémon have a similar BST to sub-legendaries, but are otherwise normal pokémon. All pseudo-legendaries can breed, evolve twice, and their final evolutions have a BST of 600.
The pokémon in this group are: Dragonite, Tyranitar, Salamence, Metagross, Garchomp, Hydreigon, Goodra, Kommo-o, and all of their evolutionary families.