"The Black Cat" by Edgar Allan Poe resembles "The Strange Case of Dr. Jekyll and Mr. Hyde" in the fact that at the beginning of the story, you have a calm, friendly, kind man, but by the end of the story, he is a villianous, rage-filled murderer. That outline holds true for both stories. In "The Black Cat," Poe has the narrator describe himself at the beginning of the tale as having "docility and humanity" in his "disposition," and as being very happy in his state in life. However, like a bad biography of a celebrity rock star, the narrator too was not free from vices and addictions. The text mentions that through "intemperence," or alcholoism, his personality suffered a "radical alteration for the worse." He became violent, ill-tempered, and abused both his wife and all of their pets. By the end of the story he has resorted to outright murder.
Compare this to "The Strange Case of Dr. Jekyll and Mr. Hyde." In this tale you have the friendly scientist, Dr. Jekyll, who is decent, humane, and giving. But, through the use of his chemical formula--much like the alcoholism in Poe's story--he turns into the violent and savage Mr. Hyde. Slowly, he deteriorates and his savage self takes over, and he commits horrific acts under the influence, just as the narrator of Poe's story did.
I hope that those thoughts help a bit with those two comparisions; good luck!