It has to be horrifying and intimidating... that would be my premise..
Ancalagon maybe more than Galaurung...
Glaurungs Horror maybe should be exactly thar he is NOT completely unhuman... i think the humanesque reminiscence in him should be the horrifying element..
While Ancalagon... to me the exact opposite! He is Leviatan and Behemoth above everything else... a different category of fear!
Indeed. When Bilbo calls Smaug "Chiefest a d Greatest of Calamaties", it is an echo of Ancalagon the Black.