Like normal objects, an animated rocking horse has Hardness. This Hardness reduces any damage it takes by an amount equal to the Hardness. Once the animated rocking horse is reduced to less than half its Hit Points, or immediately upon being damaged by a critical hit, its construct armor breaks and its Armor Class is reduced to 13.

Construct Armor (Hardness 5)

Princess Sunset Strides up to double its Speed and can move through the spaces of creatures of size Small or smaller, Trampling each creature whose space it enters. Princess Sunset can attempt to Trample the same creature only once in a single use of Trample. The monster deals the damage of the headbutt Strike, but trampled creatures can attempt a basic Reflex save at DC 14 Basic Reflex
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Trample

Princess Sunset Strides up to double its Speed and can move through the spaces of creatures of size Small or smaller, Trampling each creature whose space it enters. Princess Sunset can attempt to Trample the same creature only once in a single use of Trample. The monster deals the damage of the headbutt Strike, but trampled creatures can attempt a basic Reflex save at @Check[reflex|dc:14|basic]
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@Localize[PF2E.NPC.Abilities.Glossary.Trample]