Call Him Demon — Henry Kuttner

In the 1920s a group of children staying at their grandmother's house realize that one of the uncles who lives there is not a real person, and only arrived there a few weeks before. He is able to exert some kind of mental influence over the adults of the household, which makes them believe he has always been a member of the family. The Wrong Uncle, as they call him, is a kind of projection -- or, more precisely, a detachable limb in human form -- of a creature which lives in a cavern deep beneath the house, accessed by a portal in the attic. The thing has only two emotions: hunger and satiety, and it only eats raw meat