There are several mobs in Entwood that are only there when
there's a quest for them. Sounds like the slugs come under
this heading. Either Quickbeam or his Huorn will give you the
quest for those mobs that day ... or they won't.
There's always the three crafting instances, if you craft, or even
if you don't craft but want to work toward level cap or Entwood
Kindred. The mobs in those instances vary from day to day
(sometimes, from hour to hour), but there are always mobs.
Eruanne - Shards of Narsil-1 - Elendilmir -> Arkenstone