From our perspective, in 2020, we have a large and quite exhaustive view of the fauna around the world. In the past, each new island could give many surprises to first people landing on.
From continent to continent, island to island, there was more specific biogeographic realms. Their fauna could be drastically differents. I don't think I have to go further in this point. However I would like to add some historical content about the meeting of animals and humans.

In the history of humanity, each time people travelled to a new island or continent they discovered new species. Each times, the species faced changes and sometimes human predation and often simultaneously of mutations of their environments. Here are some examples : 

In America :

- Before the arriving of the conquistador, there was no horses in America. In the plains, there were millions of bisons of them rather than some few ones in Europe. 

- The first human arriving in North America may have extinct the Megalonyx, the giant beaver (3m long) and then we the first human arrived in South America they may have extinct the giant megatherium. The explanation of their extinction coud be due to environmental changes but here is not point of this thread.

In Madagascar :

- Megaladapis, Palaeopropithecus, and other giant lemurs disappeared soon after the arriving of the first humans.

- the Aepyornis or elephant birds  which could be 3,5 m tall bird and weight  more than 700kg !

In New Zeland : 

- the Haast Eagle, the largest eagle ever existed. He was able to prey the moas (among other animals)

- the moas (one of the species could reach 3,6 m tall and 250 kg) 

They both disapeared in the XIV -  XV century hunted by the maoris.

There are tens of other smaller species and sometimes iconic (the dodo from Maurice Island) that could be easily added to this list.

I love exploration, discovering new mountains, forests, ressources, environments and fauna. 

The idea here is not to put animals everywhere. That will make no sense to the game. I am just considering 2 or 3 possibilities : 

      - Adding new species of animals roaming on the map depending on islands, continents and environment. We can already see elephants, bear, 'antelope', birds,.... (the idea of this thread comes from these animations of animals on the map). It could be interesting to add bisons, giant birds, .... some of them moving in herds. And why not, even whales in the ocean ?

      - Giving some narrative events/quests relative to fauna when you discover a new continent / island.
When the Maoris arrived in New Zealand, how did they choose to deal with an eagle able to easily kill a man? --> Giving him a religious glorification? Hunt them all ? leave it alone?  

We can easily imagine a dozen of events linked to these fauna. It may improve merchant, culture, etc...

So a little bit of flavoring with fauna?

This is an interesting way to keep some of the exploration feel of the Endless series, like anomalies, while still remaining true to the reality of Earth. I also wouldn't be opposed to a fictional animals mode, since we're already playing a fictional history game.