Horse / Donkey / Mule health points is 26 ♥ by command to spawn.
Use command:
/summon EntityHorse ~ ~ ~ {Type:0 / 1 / 2}
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Skeleton horse / Zombie horse health points has always been 26 ♥.
If you provide any NBT data all missing data will be set to default.
Confirmed with every single type of horse, including Zombie and Skeleton variants.