In a comment I asked:
"animals that can be trained to accept spells are rare." - I think this point is easy to misinterpret in a variety of ways. For instance, is it the type of animal that can be trained which is rare? (Horses, but not Cows, Deer, or Zebra) Is it the difficulty of training particular types of animals? (anybody can train their dog to sit. To train a dog for much fancier things can require a smart dog and a lot of work - but people would probably do that much work for magic horses) Or is there some individual trait for some animals, or "Magic-proneness" any type of animal could have?
I believe this, along with the cost of manpower, is what will ultimately drive the decision.
Why? Because unless magical-acceptance is completely random, we will find a way to make them "not-so-rare".
Certain types of animals: Horses would presumably be one of them. Horses would be used all the way.
Training is difficult: New occupation of magical horse-training emerges. Horses would be preferred, but only available in locations with horse-trainers. Certain traits are easier for training, and are breedable (next point).
Animals require some kind of trait or intelligence which isn't random: Horse breeding takes place, just like it did in the past. At first, humans are more used. But as more horses are bred which are capable of magic, they become cheaper and more available.
Completely random - magical horses will always be rare for magical reasons, magical horses are only for the incredibly rich: Depends on local cost - which is more costly, magic or basic horse? Look to the next section:
Consider the following for each physical labor situation:
- How many humans/horsesmagic horses/normal horses would you need?
- How many of each could you possibly purchase/hire, and supply? (food, gear, etc.) How expensive are the supplies?
- Can you effectively run that many at once or watch over that many? How many men would you need to hire to supervise? (or do they not even need to be worked or watched?)
- After figuring out the cost of multiple compositions of humans+horses, Weight the decision more-or-less towards whichever method requires less direct work for the land-owner.