Well, dogs ate pretty much what we ate. Meats, veggies, fruits, bread and anything they could find outside.
In medieval times, royal dogs lived in posh kennels and were fed a rich stew of lungs, livers, grains and vegetables. These were hard times and feeding dogs quality food was hard for most people as they could barely feed themselves.
Domesticated dogs became status symbol and the lower class started re-purposing horse meat and selling it as dog food to wealthy dog owners.
As society progressed and more people started keeping domesticated dogs as pets, owners began paying more attention to their dogs' food and their health.