The story goes that at the beginning of time, the great ancestor of all starslugs slimed ponderously across the great vault of heaven, and left behind a sticky, phosphorescent trail of stars. After a few nights of this, the night was as bright as day (and with a regrettable tendency to drip) and nobody could get any sleep. Needless to say, steam-cleaning an entire sky is not something you want to pay for on a weekly basis, and anyway, you never get all the stray stars out, and pretty soon your nice pristine inky vault of heaven has little white flecky bits all over it, but that's life for you.
So the gods banished the starslug from heaven, and it made its slow and ponderous way down to earth, leaving a last slime of stars behind it, which on a clear night, you can still see in a broad band across the sky.** Its descendants made their home in the caverns underground, and they generally don't get more than a few inches long. But somewhere, the mouse-tailed hunters claim, in some long forgotten cavern, is the great ancestor of all starslugs, a mile wide and ten miles long, sleeping under the earth until the day when all the stars go out.
I have no idea where that came from. I was thinking about a painting, which had the title "The Starslug Snuffer" but no accompanying visual, and it turned into...um...this...instead. Eh, go figure. I won't quit the day job.
* Generally lumpy, fangy residents. Removing starslugs from trollholes and boggledens can be a profitable full time occupation for a hunter with a strong stomach and suitably adamantine spine.
** There's another legend that the gods had a dispute with their cleaning company, and the Great Mother of all Unions gets involved, which is the reason the Milky Way is still there, but we won't get into it here.