Most animals can be encountered in skeletal or zombie varieties when visiting a haunted, sinister or terrifying biome. The type of undead animal is the same as the animals which would be found in a normal biome of the same type, and they are encountered in the same size groups.
Undead are highly aggressive and will attack and chase any living creature that approaches them. Zombie animals move slowly, so dwarves can usually outrun them. Skeletal animals, however, are both strong and extremely fast, making them very dangerous.
Undead creatures do not need to breathe, rendering water moats largely ineffective (though magma will likely still be effective). All undead gain the BUILDINGDESTROYER tag at value 1, allowing them to deconstruct wooden doors, windows, statues, archery targets, and wooden supports.
Zombies start with brown and grey wounds to several of their organs; skeletons start with grey (missing) wound status on all their organs. Both types appear to be immune to critical hits against organs, making crits with piercing weapons meaningless, although they can still be dismembered by an axe or knocked across the screen with a hammer.
When defeated, a zombie leaves a rotten corpse which should be disposed of to avoid miasma. It will eventually rot completely to leave a pile of bones. Skeletal creatures leave a skull and bones, which can be used immediately by a bone carver.
Undead trees show on the map as Dead Acacia, but an undead Dead Acacia Sapling will still mature into a full grown tree. After cutting down, the logs are indistinguishable from those cut from a living tree. Undead shrubs and grass are useless.Verify
If you find skeletal elephants or other exceptionally large creatures... either lock your fort down or consider abandoning.