What do cats symbolize in "The Black Cat" and "The Cats of Ulthar"?
Cats symbolize wisdom and justice in both tales. In Poe's "Black Cat," the narrator begins by describing his love for creatures. But, "the disease of alcohol" begins to consume him, and he takes it out on his cat which seems to return later in the form of an identical cat who will not let the narrator forget his cruel acts toward his pet. In the end, the narrator thinks that he has gotten away with murdering his wife and hiding her body within the wall of his home, but the police hear the cat in from behind the wall and discover the narrator's dead wife.
In Lovecraft's "The Cats of Ulthar," the narrator stresses that
"in Ulthar, which lies beyond the river Skai, no man may kill a cat."
Thus, when a cotter and his wife kill cats, especially the prized cat of a little orphan boy, the cats get revenge upon the couple by attacking and eating them at night.
In both stories, the cats are black and seem to be immortal (similar to Egyptian beliefs). They try in their own way to provide warnings to their master or oppressor, but neither listens, and justice is served when the cats get revenge for the wrong done to them or to others--hence, the fear of cats.