this might help:
https://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_breeding#Breeding_rate
also found:
https://pokemondb.net/pokebase/187538/just-chance-that-pokemon-other-while-breeding-theyre-group
Gemini sez:
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Your assumption is correct! For the purpose of calculating breeding compatibility and egg production chances, a Bulbasaur and a Ditto count as different species.
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Even though Ditto can morph into any breedable Pokémon, the game’s breeding logic always registers Ditto as its own distinct species.
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Bulbapedia does fully support and confirm that exact mechanic. Here is how the page structures it:
The Definition of "Species": Under the Breeding rate section, Bulbapedia notes:
"The chance of any two Pokémon producing an Egg is determined by their species (two Bulbasaur are the same species, while Bulbasaur and Ivysaur are not) and their Original Trainers."
Ditto is Classified Separately: Throughout the guide's charts (such as under the What will hatch section), Bulbapedia explicitly separates "Same species breeding" (e.g., Krookodile + Krookodile) from "Ditto breeding" (e.g., Dragapult + Ditto).
Because Ditto's species is "Ditto" and your other Pokémon's species is "Bulbasaur," the game's code reads them as two entirely different species. Therefore, when you look at Bulbapedia's breeding rate table, a Bulbasaur and a Ditto will always fall under the "Different Species" rows.
