Honestly, Papyrus would be difficult to identity too, were it not for his fairly notable voice, demeanor, and the fact that he's still pretty obviously a skeleton. Whether or not Papyrus is scary may be a bit subjective but he's pretty sure that's not the problem.

"I think... something's been done to them. They're all wearing collars..."

In his world, at least, that has a pretty specific connotation.