Fishmen are the original citizens of the Ancient Kingdom that was taken by the sea in the Void Century, they evolved to fishmen through the years to survive.
Space animals were the original AK citizens, the likes of which we've seen in Enel's cover story.
When the AK was sunk, those who "stayed behind" survived by evolving gills and breathing underwater.
Those who sailed away, grew different types of fur, becoming the current Minks.
Once the flood was over and there appeared new land to be populated, Minks further evolved into humans, whom Minks themselves on Zou defined as "hairless monkeys".
OP at is core is the fight between outer space animal-like creatures and metaphysical animal-like demons.
