Overall, some pretty good beings, especially in concept. A few things need looking over, though: for example, the mer-octopi have an effective straight-line speed of about 200' thanks to a lack of a limit on their "jet" ability. You can certainly mine worthwhile ideas, though.

Like Volume One, Volume Two of the Fell Beasts series is a nice package of diverse and interesting creatures, beautifully illustrated. I particularly liked the deadsoul elementals this time around, as interesting higher-CR monsters that aren't "another demon" tend to be few and far between. Very solid.