Mistwraiths are creatures composed only of soft tissue, who consume the bones of several other creatures, which they then use as their own skeleton. They become fully conscious Kandra when given a blessing.Edit
In book three, it's revealed that Mistwraiths were the Terris Ferrochemists of old. The Lord Ruler recognized that his friends, the packmen he went to the well of Ascension with, could one day challenge him if they got Allomancy. When the Lord Ruler had the power of the well, he made a deal to give them immortality - in exchange for their humanity.