Branching limbs is for the demon generator, already planned. That one will basically take your average FB or Night Creature, do you one better, and make them roam in packs. This one is for people who are tired of macaque and wolves, and want to see some interesting wildlife.
Sadly you cannot add true undead creatures, like zombies and skeletons, but you can make creatures that resemble the undead, like have a creature that has organs that don't actually function. You can pierce the heart, but it won't matter. Skeletons are also possible, but bone isn't the strongest material, so using copper or bronze for bones would give it a better feeling. Their thin size makes them easy to smash.
Fish have gas-bladders, used for swimming up and down more easily, but that's a bit different. The main thing is that floating has never been needed. It's much easier to crawl somewhere, or fly with wings. I think a large expanse of desert would promote the use of bladders though, similar to the way that vultures will glide for hours at a time, except much easier because you just float and stay there.
As a side-note, I was thinking about making "toy creatures", similar to "toy poodles". These creatures would be high pet value, low birthrate, and easy to kill. They'd have like, glass bones and paper skin, so any trauma just rips them apart, but you could breed them and sell them at a profit.