Size and Weight:
Despite its name (which means "chicken mimic"), it's possible to overstate how much Gallimimus actually resembled a chicken; unless you know many chickens that weigh 500 pounds and are capable of running 30 miles per hour, a better comparison may be to a light, low-to-the-ground, aerodynamic ostrich. Gallimimus was the prototypical ornithomimid ("bird mimic") dinosaur, albeit a bit larger than most others of its kind, such as Dromiceiomimus and Ornithomimus.
Gallimimus has been featured prominently in Hollywood movies: it's the ostrichlike creature seen fleeing Tyrannosaurus Rex in the original Jurassic Park, and it also makes smaller, cameo-type appearances in the Jurassic Park sequels. Considering how popular it is, though, Gallimimus is a relatively recent addition to the dinosaur bestiary. This theropod was discovered in the Gobi Desert in 1963, and is represented by numerous fossil remains, ranging from juveniles to full-grown adults; decades of close examination have revealed a dinosaur with hollow, birdlike bones, well-muscled hind legs, a long and heavy tail, and (perhaps most surprisingly) two eyes set on opposite sides of its small, narrow head, meaning that Gallimimus lacked binocular vision.
There is still serious disagreement about the diet of Gallimimus. Most theropods of the late Cretaceous period subsisted on other animals (other dinosaurs, small mammals, even birds venturing too close to land), but Gallimimus may well have been omnivorous, and one paleontologist speculates that this dinosaur may have been a filter feeder (that is, it dipped its long beak into lakes and rivers and snatched up wriggling zooplankton). We do know that other theropod dinosaurs, such as the feathered therizinosaurs, were mainly vegetarians, so any theories about Gallimimus can't be easily dismissed!