pf2e.fyi
From Pathfinder War of Immortals, which isn't enabled.

Behold, a Pale Horse

FEAT 14
MYTHIC
Archetype Apocalypse Rider
Prerequisites Apocalypse Rider Dedication

Your apocalypse mount becomes worthier of an eventual Rider of the Apocalypse. Your apocalypse mount gains one specialization of your choice.

Special You can select this feat up to three times. Each time, add a different specialization to your apocalypse mount. Your apocalypse mount can't have more than three specializations.
Pathfinder War of ImmortalsORC