Gallimimus mongoliensis is a species of large ornithomimid theropod dinosaur that originally lived in what is now Mongolia during the Late Cretaceous period, about seventy million years ago (mya), but has since been brought back from extinction by SciiFii and now live in Cretaceous Park in Sacramento, California. The adults are generally about 6 metres (20 ft) long, 1.9 metres (6 ft 3 in) tall at the hip and weigh about 440 kilograms (970 lb). Just like its relative Ornithomimus, it has a thick coat of feathers covering most of its body except its lower legs or its beak. The head is small and light with large eyes that face to the sides. The snout is long compared to other ornithomimids, although it is broader and more rounded at the tip than in other species. Gallimimus mongoliensis is toothless with a keratinous (horny) beak, and has a delicate lower jaw. Many of its vertebrae have openings that are pneumatic (air-filled). The neck is proportionally long in relation to the trunk. The hands are proportionally the shortest of any ornithomimosaur and each have three digits with curved claws. The forelimbs, which are in fact wings, are weak while the hindlimbs are proportionally long.
As an ornithomimid, Gallimimus mongoliensis is a fleet (or cursorial) animal, using its speed to escape predators; its speed is generally between at 42 and 56 km/h (29–34 mph). It has a good vision and intelligence comparable to ratite birds. Gallimimus mongoliensis lives in groups for safety in numbers. It is normally a herbivore and mainly feeds on various tough and soft plants, but may also feed on insects and smaller vertebrates and occasionally feed on carrion and/or eggs to supplement its diet.