Pokemon Sun and Moon?

[Dolphman]

There's really nothing duck or bird about Psyduck and Golduck. For starters, they have claws, long thick tails, and are based on the Kappa.
Most realism artists do tend to go with making them resemble platypuses (yes that's the correct plural) more than ducks.
A platypus is a monotreme, which are beaked mammals that lay eggs. Echidnas belong to this group as well.
You know the Belgian comic creature, Marsupilami? They're a fictional species of monotreme. 

I do agree we need a region based on Australia, but the hard part would be digging into our native fauna besides the "stock" animals.
And it's best that they don't include invasive species that are legal to kill here, like cane toads, foxes, rabbits, Indian myna birds, and cats.

One good example of one of our animals that not many know of. Say hello to the Christmas beetle.

It is a native species of scarab beetle that's only active during our summer (December to February) hence the name.

For an idea of what prehistoric Pokemon they could base them on. Some of these could be good examples.

Spoiler

Meiolania, Australia's only native species of tortoise. As you can see, it has horns on its head and a tail covered in spikes.

Palorchestes, AKA the Marsupial Tapir.

Megalania. A giant goanna so big, it puts the komodo dragon to shame.

Thylacoleo carnifex. Nicknamed the marsupial lion due to its size and skull shape. It has teeth unique to marsupials too.

A collection of our native dinosaurs.

Diprotodon. Think of a wombat-like mammal as big as a rhinoceros.