Films: Slugs (1988)
Location: Forest/Lake/Civilized area/Underground
Height/Weight: That of average slugs.
Summary: A lot of people would probably scoff at the idea that creatures moving only slightly faster than their snail counterparts would pose anything resembling a threat. Try saying that to the hundreds of them barging down your house and coming after your flesh.
History: An overabundance of pollution has mutated hordes of slugs in the worst way possible. Now, they have jaws and teeth all the better to gnaw at people with.
Notable Kills: One accidental slug salad later, a guy literally has them tearing out of him.
Final Fate: Eventually, the slugs' hideout in the sewers is discovered, and the place is blown to cinders. However, at least one slug remains...
Weakness: Anything conventional.
Scariness Factor: 3.5-Admittingly, most of the deaths could have been avoided if the victims exercised a bit more carefulness. But make no mistake, those who do find themselves being overpowered by these toothy mollusks will find that death won't be clean at all...
Trivia: -The novel this film was based on actually got a sequel, named "Breeding Grounds", in 1985. A film for that was canned.
-Slugs are actually quite opportunistic with their diets. Most of it is plant matter, but many will also settle for rotted flesh.