Horses won't get any stronger just by eating. A lot more goes into raising 'em right. Stop by the stables and you'll see what I'm talking about.
Raise the horses well. They are valued friends whom you may, at times, rely on to carry you into battle. Visit the stable and talk to the stable hand to learn how to care for them properly.
  1. Accept the quest from Jeralt.
  2. Train a horse.
  3. Report to Jeralt.
Do you have a feel for training horses now? If you want them to get stronger, you'd do well to look after them from now on.