Was Spinosaurus the only sailback dinosaur?

No, some other dinosaurs also had sails. One was Irritator, a relative of Spinosaurus, which had a similar, but smaller, sail. Another was Hypacrosaurus, which was a quadrupedal herbivore that had very high spines on its back. One animal commonly referred to as a dinosaur, Dimetrodon, was not a dinosaur, but it was a prehistoric reptile with a sail on its back.