Let's say the Pleistocene goes on normally outside of one detail: Horses never go extinct in the Americas, and eventually go on to be domesticated. Obviously this would have massive butterflies in the New World and essentially make it unrecognizable, but that's a thread for another day.
Instead, I'm wondering what changes would carry over to the Old World. Would things carry on exactly the same up until contact? Would there be climatic changes that could heavily affect them, or would this only lead to very subtle changes?