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Ever seen a newborn foal's hooves? In humans, eponychium is the white tissue at the bottom of out fingernails —in horses, it's, well… horrifying tentacle-looking tissue called deciduous hoof capsules that keeps the foal's hooves soft and non-harmful during birth. Which is always a plus, to not be stabbed with hooves from the inside :D
And seeing as how unicorns have very sharp and harmful-during-birth protrusions on their foreheads, it's not completely unrealistic for them to also develop something similar there…
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Just a fun, creepy-looking mini-headcanon for the spooky season =P Decided to draw newborn Luminous, because might as well rather than design a completely new foal. He's all wet and slimy because he's just born; so recently he's still got his hoof- and horn-capules still intact —they later fall off as they dry.
Ha! Nah, don't worry, horn-capsules aren't actually canon to my nextgen-verse. No, my newborn unicorns just have very flat bumps on their heads that grow into a proper horn-shape in about a month after birth. No protection needed :)
…Now, the hoof-capsules though, that's 100% canon.