Ornithomimus is a slender and very fast bipedal theropod dinosaur with long hind limbs and a large body, as well as a toothless beak. These bird-like, swift-footed dinosaurs measured nearly 4 meters in length.

The namesake of the ornithomimid family of dinosaurs, this species was first described in 1890 based on a partial hind limb dating to the Late Cretaceous period, found in the Denver Formation. In 2015, paleontologists redescribed the species using the same bone fragments. The name Ornithomimus literally means "Bird Mimic", due to its ostrich-like structure. Throughout history, there has always been disagreement over the diet of Ornithomimus; some paleontologists believe they used their signature bony beak to either pluck vegetation or attack prey.