So a friend is working on a worldbuilding/rpg project and I'm "beta-testing". He drew up an earth-like map and set the players start locations. Then I ask him about fauna. Will it be modern earth? Prehistoric megafauna? Fantasy monsters? Dinosaurs? To which he promptly replies, "yes". After some needling, we agree that monsters and dinos will be rare and mostly regional. But he does want megafauna to be more common and live alongside normal modern species. My main question: is this feasible?
Or more specifically: Do we know much about species overlap between extant and extinct species? Could there be enough niches for everyone to survive without over-crowding the world with such a menagerie? What about cases like the many giraffe cousins, did they all live in the same area with different niches, did they compete or did they live apart?