From Imperial Wiki
A space slug is an enormous, worm-like creature known to inhabit asteroid fields. Virtually nothing is known about their life cycle, although they are known to harbor mynocks in their digestive tracts. One of these monsters inhabited an asteroid in the Hoth system; when the Millennium Falcon took refuge in its belly, the beast attempted to prevent the ship's escape.
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Space slugs can grow to 800 meters or more in length.
- Space slugs on Star Wars.com