You've trained with your shield to defend both yourself and your mount.
Archetype
Feat type
Archetype Feat
Level
6
Traits
Archetype
Prerequisites
Cavalier Dedication
You've trained with your shield to defend both yourself and your mount. When you Raise a Shield while mounted, both you and your mount gain the shield's circumstance bonus to AC. If you have the Shield Block reaction, you can use it when your mount would take physical damage, as long as you're riding your mount. If you do, the shield prevents your mount from taking damage instead of preventing you from taking damage, following the normal rules for Shield Block.
Source: Pathfinder Player Core 2, p. 193
Bring Mounted Shield into a Pathfinder 2e game with The DM Screen.