They can feel it in their bones
Skelekenesis
In DnD it’s because most undead are damned and tortured souls enslaved by dark magic to the bidding of their twisted masters.
The skellingtons are looking at you, the real you, the core of your very existence with their soul eyeballs. And they hate you, because you are alive, and they aren’t just dead, they’re being tortured every moment of their existence.
But, hey, tell me more about your perfect medieval civilization built on raising zombie field hands.
/Jokekilling
Skelepathy
Bone sense
It’s wet in you. They have to rescue your skeleton from drowning.
“Your bones are wet in you.”
Unpleasant. Mmh. No, no thank you.
So the ultimate counter to skeletons are octopuses.
Octopuses have beaks. Are beaks considered part of a skeleton?
Isn’t the proper plural term octopi?
Octopodes, actually.
Technically they’re all “correct”, just that octopodes is the most correct and coolest sounding