This quote from Scythemantis' review of the Dullahan sums it up well, I think:

"Why do we forget the grotesque in favor of the streamlined and threatening? A headless corpse is "scary," kind of like how a killer robot or an angry grizzly bear is scary, but a headless corpse with a cartoonishly exaggerated, impossibly huge mutant head rolling around with it is insane. A rotten horse with a normal sized body and a fifteen foot face is disturbing. Monsters don't need to be realistic or even serious to be frightening - the more outlandish, the more impossible, the more ridiculous, the more they'll leave you questioning your very sanity, should you even escape with your brain still attached."

(Emphasis mine.)

The parasite by axxyn-d6knoo5

Drawn by Axxyn on DeviantART.