What type of creature does the narrator find inside his prison?

In the story, the narrator discovers a large, mysterious creature that resembles a rat, but with distinctly unnatural features, including an elongated body and an unsettling intelligence. This creature becomes a source of both fear and fascination for the narrator, as it embodies the horrors of his confinement and the psychological torment he experiences. Its presence serves to amplify the themes of isolation and madness within the narrative.