>>4125631The book “Sinnoh Folk Story 3” you could find in Diamond and Pearl gives you the answer;
Japanese to English tranlation:
"There once were Pokémon that married people.
There once were people who married Pokémon.
This was a normal thing because long ago people and Pokémon were the same."
English:
"There once were Pokémon that became very close to humans.
There once were humans and Pokémon that ate together at the same table.
It was a time when there existed no differences to distinguish the two."
>>4125638If I remember right in the anime and in some places in the games you could find some unnamed fish, birds and insects based on real animals.