Shadow Vermin appear as small, black, squid-like creatures that appear in areas overcast by Twilight, which attack by flailing at Link. They are first encountered in the Castle Town Sewers while escaping with Midna, shortly after Link was transformed into Wolf Link. Fighting them can be tricky, as they will attack while swimming across the water, and can sometimes be hard to spot until they get very close. They are quite fast, and can attack when least expected. Their behavior and small appearance is similar to that of Rats.
Shadow Vermin also make an appearance within the Palace of Twilight. The only difference is there is no water for them to skim across.
- Encyclopedia (Dark Horse Books) pg. 202