Mainly all the bird-like dinosaurs, even something as large like Allosaurus was stealthy enough to avoid detection and it got into some crazy fights. And the key to being a Ninja you must swift and cunning, theropods like Allosaurus exhibited this.
Solmonster, the Daily Phantom wrote: Hmmm... I'm not sure any dinosaurs could actually fight like Ninjas.
Anyway, we don't know because we weren't alive at that time.
That is because Dinosaurs are the DeFacto true ninjas. These animals are stealthy, and great speed, a lot of them use the environment as their tools, to trick prey or predators, and some of them have hidden abilities that make them dangerous. And if see birds today; dinosaurs were the first martial artists and are able to balance their own weight, which is something like T.Rex and Acrocanthosaurus could jump or perform mating rituals probably like birds today and not break their bones on first impact.