Euoplocephalus

Euplocephalus (meaning "well-armored head") was a large armored dinosaur, measuring about 20 feet (6 m) long; it weighed about 2 tons. Its wide head had a horny, toothless beak, small, peg-like cheek teeth, bony horns, and a small brain. Among its contemporaries were the large meat-eating "Tyrannosaurus" and "Deinonychus".
Fossils from more than 40 individuals have been discovered in Alberta, Canada and Montana in the United States, making "Euoplocephalus" the best known ankylosaurid. This includes 15 skulls, teeth, and a few almost-complete skeletons, found with the armor still attached. Individual armor plates are the most commonly found element from them.