Another aquatic demon. Who'd have thought a book called the Fiend Folio would have so many different outsiders in it?
Skulvyns are large-sized, with a modest 4 RHD. They're strong (18 str), dextrous (16 dex), tough (15 con), and as dumb as your average fridge (5 int). Underwater they are impressively fast, with a 50 ft. swim speed, but on land they're resigned to crawling at a mere 10 ft. rate.
Natural attacks are an 1d8 bite, two 1d6 claws, and a 1d4 tail lash. The last of those also inflicts bleeding wounds, for all that those're worth.
DR 5/good is a nice little buffer to have. SR 12 is similarly neat. Scent is moderately useful. The skulvyn's core trait: a slow aura that affects all living creatures that come within 30 ft. The 4 round duration is unimpressive, but at least making allies immune is easy.
The terrible land speed, bad mental stats, and utter lack of thumbs, hold this back from being a solid brute. I'm going to go with -0 LA on this one (approaching +0), though in an aquatic-heavy campaign it's probably a viable pick.