Starter Pokémon are Pokémon you get to choose when you create an account on Pokémon Vortex.
The options are in groups of four, per region that consist of the region's 3 expected starter Pokémon and a fourth starter Pokémon which the selection process is unknown.
Before the Pokémon Vortex v4 update, a poll was held for players to get to choose what the fourth starter Pokémon of the Alola region would be.