A follow-up to Dungeon Jellyfish, I was reading more about weird animals.
The first article covered animals with amazing disguises. There were:
A spider that looks like bird poop (somewhat similar to the carrion spider I wrote about long ago);
Caterpillars that can make themselves look like miniature snakes (by semi-retracting their head into their body, they produce the wedge-shape of a serpent's head and spots on the side look like eyes); Caterpie from Pokemon is one of these caterpillars; the Y-shape on its head is supposed to resemble a snake's forked tongue
Walking sticks, Leaf insects, and dead leaf butterflies are damn near invisible in natural settings. Walking sticks actually sway back and forth to mimic branches in the wind. Leaf insects not only resemble leaves, they actually resemble leaves with a little bit of damage (like dead edges where caterpillars have eaten away the leaf). Dead leaf butterflies can simply fall to the ground and look exactly like dead leaves due to the markings on their wings (they can also cling to trees and look like dead leaves that have not yet fallen).
There is a clam whose "lips" resemble a minnow (I believe it is actually an egg sac). When a predator attempts to eat the minnow, the eggs burst open to release burrowing, parasitic larvae who then infect the predator.
There are spiders who look exactly like ants. One species has very large fangs/mandibles for mating rituals and fighting; however, since the ants it mimics do not have large fangs, it disguises them by looking like the front half of a second ant. Imagine fighting a giant ant coming up through a hole in the ground (with another ant visible behind it) when suddenly the ant splits in half right down the middle to reveal that it is, itself, a giant pair of mandibles.
The stonefish looks exactly like ... a stone. It sits motionless on the bottom of the ocean and sucks shrimp and small fish into its mouth. It also has poisonous spines across its back that can kill a grown man in two hours. A living stone in the floor that eats vermin and can poison adventurers who step on it is a jerk.
The king of disguise, however, was the Indonesian mimic octopus. It is able to bend and shape and recolor/repattern itself to resemble a number of other creatures, such as hiding in a hole and using two of its tentacles to mimic a deadly sea snake or pulling all its tentacles around it to resemble a foul-tasting flatfish.
You could take this octopus pretty much at face value (changing it to a land-dweller who just mimics other deadly creatures; Spot check DC X to notice the creature, DC Y (Y>X) to realize the mimicry), or dial it up (allow it to utilize some of the attacks of the mimicked creature (poison pneumocyst in the tentacle doubles as a snake's bite), or dial it WAY up (make it an actual shapeshifter).
Some of these creatures just follow in the footsteps of classic dungeon mimics, like lurker above, trapper, darkmantle, piercer, and, of course, the eponymous mimic. As such, their use must be deliberate in order to maintain effectiveness and not bog down gameplay by making the PCs paranoid.