Sometimes the Teen Titans didn't face a human opponent, or even a humanoid opponent. Sometimes they faced things much more...strange. Such as Cinderblock, a being literally comprised of cinderblock material.
Ironically, there's virtually no backstory as to how or why this being came to exist. But, he kept showing up in the series, and for good reason, as he's a tough opponent. In fact, Slade repeatedly used Cinderblock to either defeat or distract the Titans.