Hell horses will become unruly when they become a certain age within the Old age stage.

You can counteract aging of animals by making electrum runes and using them on your animals. It may take several runes to reduce your animal to a desired age.

If you don't want to gather rift materials for electrum runes, the best way to keep young hell horses for your vehicles is to breed them.

Caring for an animal can prolong its life indefinitely, but there is still a small chance that it will die eventually. Cared for animals tend to live for years in real life time.

• Charm or tame the hell horse.
• Care for it.

NOTE: Being on a settlement may prohibit you from doing this. You may have to brand the animal first.