The history of "Ornithomimus" classification, and the classification of ornithomimids in general, has been complicated. The type species, "Ornithomimus velox", was first named by O.C. Marsh in 1890, based on syntypes YPM 542 and YPM 548, a partial hindlimb and forelimb found on 30 June 1889 by George Lyman Cannon in the Denver Formation of Colorado. The generic name means "bird mimic", derived from Greek ὄρνις, "ornis", "bird", and μῖμος, "mimos", "mimic", in reference to the bird-like foot. The specific name means "swift" in Latin. Simultaneously, Marsh named two other species: "Ornithomimus tenuis", based on specimen USNM 5814, and "Ornithomimus grandis". Both consist of fragmentary fossils found by John Bell Hatcher in Montana of which it is today understood they represent tyrannosauroid material. At first Marsh assumed "Ornithomimus" was an ornithopod but this changed when Hatcher found specimen USNM 4736, a partial ornithomimid skeleton, in Wyoming, which Marsh named "Ornithomimus sedens" in 1892. On that occasion also "Ornithomimus minutus" was created based on specimen YPM 1049, a metatarsus, since recognized as belonging to the Alvarezsauridae.