If every region had a fourth starter like Pikachu was for Kanto, which Pokémon would it be and why?

Before we start this, I feel that there should be a few criteria laid out:

The ‘fourth starter’ must be an already-existing Pokémon that was introduced in the same generation as the region it’s a starter for. It must also not be a Fire, Water or Grass-type, at least in its first stage.

The fourth starter must be able to evolve at least once (since Pikachu and Eevee obviously evolve once) and doesn’t necessarily have to evolve through just leveling. However, it cannot be a trade evolution.

The fourth starter would ideally be a Pokémon that can be found fairly early in-game (for instance, Pikachu can be found in Viridian Forest in the original Kanto games). Also, unless there’s no other options (ie if there’s a really small native regional Pokédex), the fourth starter cannot be a regional bird.

So with that out of the way, what do you guys think would be the hypothetical ‘fourth starter’ from Johto to Paldea?