Size and Weight:
Ornithomimus is the type of dinosaur that movie and TV directors love to show stampeding in herds across vast prehistoric plains: this theropod may have been able to run as fast as 30 miles per hour when escaping predators, and it may also have been covered in feathers. Unlike an ostrich, though, which it superficially resembled, Ornithomimus was definitely a land-bound dinosaur, and not a bird. It was closely related to other ornithomimid ("bird mimic") dinosaurs like Dromiceiomimus and Gallimimus, which may have been even faster.
Like some other human-sized theropods of the late Cretaceous period--most notably Troodon, which belonged to a slightly different family of theropods--Ornithomimus was also distinguished by its fairly large brain. While it's unlikely that this "bird mimic" was as smart as a modern ostrich (and granted, that wouldn't be saying much), this is a tantalizing clue that carnivorous dinosaurs may have been on the verge of evolving bigger brains before they went extinct.
Like many dinosaurs discovered in the late 19th century, Ornithomimus has a complicated taxonomic history. This theropod was named in 1890 by the famous paleontologist Othniel C. Marsh, based on scattered remains from Colorado; additional species accumulated over the next 50 years, some erected on very dubious evidence, until Dale Russell sorted out the mess in the 1970's and left only two species standing (the original Ornithomimus velox, named by Marsh, and Ornithomimus edmontonicus, named by Charles M. Sternberg after a near-complete specimen discovered in Alberta, Canada.