Slug

Slug is a harmless non-hostile creature that usually spawns in an open place where there is a diminished light level and any background block. If a slug becomes completely entrapped due to falling snow, sand, or a dwarf placing a block, it will die. They seem to die when exposed to too much sunlight (not proven), but do not die when crawling below water levels. When killed, they drop one slime.

This kind of slug appears in Forest, Snow and Desert Worlds. An equivalent, called small caterpillar populates the Underground World.

Appearance
The slug is somehow palish red and the shell shows a lighter version of brown as earth. When a slug dies, its empty shell remains behind briefly before fading away.

Useful tip
A world can only hold a limited number of non-hostile creatures, so you should kill slugs to ensure that more sheep, wild boars and hens spawn.