Horses were the beasts of burden back in the day, so they were very important in basic travel/military strategy. According to one documentary I once watched, we got the majority of our common day diseases from animals. Getting said diseases from animals, people who worked around animals had a stronger immune system than others. So, if anything, we'd have a slightly different immune system than what we have today. Europe would use more oxen, which probably aren't good for combat. I doubt Napoleon would ride one.