There are also skeletons of killed humans.
Skeletons come in many types. Animal types are more common, though human skeletons do exist.
Once killed, the following animals/people will leave a skeleton after being skinned and butchered:
In order to ward off looters, it is common that powerful villages will leave the skeletons of their human victims outside of their palisade/wall as a symbol of what they can and have done to people in the past.
Being around a skeleton will quickly drain your happiness. (However, please note that Happiness has no real importance yet [up to date as of World 7]).