Feeding a golden apple has no extra effect (such as adding more hitpoints) to a horse. It just acts as a regular apple.
MC-176674
Golden apples have no effect on horses
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Golden apples can be used to breed horses (normal apples cannot), and golden apples heal a horse by 10 hitpoints (normal apples by 3), speed up the growth of baby horses by 4800 ticks (normal apples by 1200 ticks) and increase the temper of untamed horses by 10 (normal apples by 3).
See https://minecraft.gamepedia.com/Horse.