[Horror...]

Bones: Weak, as the character has not eaten anything for many years.
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Head: Large and very noticeable crack on the head.
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Eyes: completely black, but the left eye has a blood-red pupil.
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Smile: The grinning grin became wider and more manic after the character's skull was fractured.
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Weapons: Sometimes depicted with a large axe, but in the original comic, the character has no weapons.

Clothing: blue jacket, black shorts with white stripes, white shirt, and blue tennis shoes instead of slippers. The shirt appears to be stained with blood.

After Horror Sans' skull was broken and some of his magic was stolen by Undyne, the character's appearance changed:
The clothes remained the same, but became dirtier from the blood.
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The jacket acquired a different shade of blue, and fur appeared on the hood.
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The smile has become wider and more manic, which may be related to psychosis or the fact that the character is in pain.

Psychological state. After years of starvation, Sans became more cruel and unpredictable, his jokes changing from annoying but funny to dark, insane, and sadistic.
Abilities. The character has become more durable, able to withstand a gaster's blaster without a scratch and instantly kill, making him dangerous in close-range combat.
Weapons. Horror Sans now has several weapons, his favorite being an axe or cleaver.