Michael

New animals are discovered IRL in places where people thought they found every animal. Not hard to do the same with Pokemon having new Regional Variants discovered in existing regions.

The secondary Starter Types in X and Y were Dark, Psychic, and Fighting. Dark, Ghost, and Fighting only appeared in Scarlet and Violet and Legends Arceus. Also, Dark may beat Ghost but, Ghost doesn't do anything to Fighting. The only advantage Ghost has over Fighting is that Ghost is immune to Fighting type moves.