I can’t say that I’ve ever encountered truly disturbing and creepy MMO mobs. I think it’s by the virtue that they are in an MMO means that they’re usually all over the place, and any initial disturbance grows lessened with familiarity.
However, I’m sure there are exceptions. The Bogeyman in Secret World is pretty creeptacular, especially his appearance in the spin-off The Park. I know that some of my friends who suffer from acute arachnophobia are all “nope nope nope” when giant spiders scuttle across the screen.
Are there any MMO monsters that creep you out? What is it about them that gives you the heebie-jeebies?