Horse Travel

You'll be able to choose your horse after meeting Markos for the first time, as part of the main quest. You can call Phobos by holding the down DPAD button. You can summon your horse in a lot of areas except for enclosed, dangerous/restricted areas, or specific locations where horse travel is not necessary/possible.

Phobos can automatically navigate and follow main road. If you have an objective marker (custom or tracked quest), you can have him follow it and bring you to the spot where the marker is located.

Phobos is immortal. He can die from jumping off high cliffs and you won’t be able to call him for a while. Leave the area or fast-travel and you should be able to call him again as if he didn’t suffer from anything. While on horseback, you can still use your weapon to attack enemies and predators. However, if an enemy or animal hits you, you may get thrown out of your horse and continue the fight on foot.