The following horses are all at full health:

These horses are all children of two horses who have full 15 hearts of health.

For whatever reason, they end up with far less health than expected (14-15), while their visual maximum represents something I would expect from two horses with a maximum health of 15 full hearts.

The horses can't be healed any more than what is seen in the images using any kind of food, nor will they heal over time on their own.

If they are hurt, then only the orange hearts will be taken into account, and not the darkened ones.

There are two things which I consider as issues here: The false maximum health, and their actual maximum health being much too low compared to their parents.