This is what I experienced so far:
Horses will be either speed or draft. So far I have had no new foals with both speed and draft traits.
The speed horses will show the new speed form if they have at least 3 speed traits. I assume the draft form will show when a horse has at least 3 draft traits.
If I breed with the old 5 speeds, they will always result in speed traits (and ofc, negatives and misc traits). This is imo because the 3 speed traits are dominant over the only 2 draft ones.
If you want to breed drafting horses, you will have to use wild horses, or horses with more draft traits than speed traits. This is based on very little experience, since I have gotten only one foal so far that has only one draft trait (strong legs). This foal has a wild horse and a zero trait horse as parents. I will have more info about this in a week or 2 with the next generation.
Speed traits: Fleeter, Lightening, very strong legs and accustomed to water (total 55 AH)
Draft traits: strong body, carry more, strong legs, ...
After reading Eletari's post, It seems it's possible to get mixed speed-draft traits. I wonder if the draft traits do anything for speed now. If so, we now need 80 AH to breed the same horses we used to breed with 50 AH.
How is this an improvement?
I feel bad about the fact that the original horse form will have no use anymore. Even our old 5 speeds are transformed into the new speed model. I miss my old horses and I wish the new model would only show when they have at least 4 speed traits, so we could still keep our original 5-speeds that we cared for for so long, and the new speed model would be more special.
I find that having HIGH AH still makes it harder to breed, despite the fact they said it wouldn't. It just makes more room for negative traits and other random traits you don't want.
Having to breed again with wild horses is also a HUGE problem on Harmony (and maybe the other northern servers as well). I have only seen one wild horse since last summer, and that was a mission spawn. (It died soon after for no obvious reason)
SO PLEASE FINALLY FIX THE HORSE SPAWN THERE.
I have had no rare trait so far. This after about 100-150 foals.