Heterodontosaurus

Heterodontosaurus ("different-tooth lizard") was a small, fast dinosaur with strong, grasping hands, long arms, and a powerfull bite. These features are, for the most part, unnecessary in a plant-eater. It does not take speed, agility, and dexterity to overpower a leaf! Heterodontosaurus's most notable feature, however, is the set of "fangs" at the front of its mouth. It is quite unusual for a plant-eating dinosaur to have teeth like this, which you'd normally expect in a meat-eater.

We know of only one other Early Jurassic group of dinosaurs -the prosauropods- where some of its members had teeth structurally intermediate between those of meat-eaters and of plant-eaters. The same may be true here. Heterodontosaurus and prosauropods both might be examples of the transition from meat-eater to plant-eater.

More real-life information: Heterodontosaurus at Wikipedia.