Hmm… a book about a Steppenwolf. What should we call it? Aha! Steppenwolf! Eureka!
Well, Hesse's thought process may have been a little bit more profound than that, but really the title is pretty self-explanatory: the main character and subject of the novel is the Steppenwolf, Harry's alter ego that torments him because he can't accept the wild parts of his nature.
We might ask ourselves why the book is called Steppenwolf and not Harry Haller. Maybe this is a hint about who wins in the fight between wolf and man. In some ways this is because Harry himself thinks of himself primarily as a Steppenwolf. By emphasizing that animal side of his nature we see exactly what it is that sets the character apart from all the other humans in the world.
There's also an ironic tinge to this novel's title. Harry Haller is a dude who had a really fixed idea about his identity: he's got a good, studious side and Big Bad Wolf side. Full stop. But throughout the trippy events of the novel he realizes that actually he contains multitudes—and that it's a dumb, sad, and limiting idea to decide that you're only one (or two) things.
So the novel's title underscores the state of the protagonist before he learns his big, cosmic lesson. It would be like if Pride and Prejudice was called Rich Dudes Are Always Jerky Snobs, or if Breaking Bad was called Cooking Meth Is A Great Get-Rich-Quick Scheme.