A "bones" is a deactivated mob, formerly referred to as "dead bones." They were originally intended to spawn underwater on Shipwrecks and attack players. They can be spawned in using spawn eggs and drop bones and skulls upon death. They have 20 health points.
"They were sailors, warriors or passengers before the ship was sunk. Now they are just some dead bones standing there with silence. What...? You say they turned around...?"
When spawned in, they must be kept in the water or the dark because they burn in sunlight. They have the ability to walk underwater instead of floating on the surface like other Minecraft mobs.