No, it would be redundant (not needed) to say "alone" and "by itself" because they mean the same thing. "The cat who walks alone." sounds more natural than "The cat who walks by itself." However, these both mean the same thing.

Ok, thank you, Dan, for your explanations! I got it!

It's not a novel. It's what is called a "short story." It is from a book by Kipling, a collection of stories to be read to children, entitled "Just-So Stories." In English, we use the word "novel" to mean a single, long, book-length story (like "Crime and Punishment.") There is also a word "novella," which means a story that is too long to be a "short story" but not long enough to fill a whole book by itself.