Which pre-historic animal would have coexist without causing imbalance with current species of earth?

Prehistoric horses were in the Americas, and I think if they returned, it wouldn’t negatively impact anything. The issue is that we have horses now, and they don’t cause any problems, so a stable population of prehistoric horses would probably be comparable and similar.

The issue might be a lack of predators, maybe, but that would be the same issues we have now if any.