Size and Weight:
The three-horned dinosaur Triceratops--which is familiar to millions of kids in plastic toy form--has the dubious distinction of being one of the last of its breed, the ceratopsians, to appear before the K/T Extinction Event that wiped the dinosaurs off the face of the earth. (See 10 Facts About Triceratops and a gallery of Triceratops pictures.) Even still, Triceratops was only the last of a long line of ceratopsian dinosaurs stretching back to the early Cretaceous period, when these ornithischians were the size of small dogs!
As fearsome as it looked, Triceratops was a strict vegetarian. Scientists think thgis dinosaur's distinctive horns may have evolved for two reasons: either for use as mating displays (i.e., Triceratops males with bigger, sharper horns were able to mate with more females), and/or as a form of defense against the larger predators of the late Cretaceous period, like Tyrannosaurus Rex. A recent study has shown that many of the contusions on fossilized Triceratops bones were caused by Triceratops horns, which implies a role for intra-species combat as well (i.e., the horns were used to defend territory or establish dominance within the herd). See Tyrannosaurus Rex vs. Triceratops - Who Wins? for an analysis of this epic battle.
One reason Triceratops is so well known is its large, bony skull, which fossilized fairly easily (and often in one piece)--a feature it shared in common with the other plus-sized ceratopsians of late Cretaceous Eurasia and North America. For this reason, complete Triceratops skulls have become prized items at auctions worldwide, fetching millions of dollars from wealthy bidders and natural history museums. (Read more here about the fossil history of Triceratops.)