As the chief zoologist on Long's expedition to the Rocky Mountains in 1819-20, Thomas Say gave the official first description of the swift fox. Following an anatomical account, Say wrote:
It runs with extraordinary swiftness, so much so, that when at full speed its course has been, by the hunters, compared to the flight of a bird skimming the surface of the earth. We had opportunities of seeing it run with the antelope, and appearances sanctioned the belief, that in fleetness it even exceeded that extraordinary animal...
Say's complete account is given in
Edwin James, Account of an Expedition from Pittsburgh to the Rocky Mountains, Performed in the Years 1819 and 1820, By Order of The Hon. J. C. Calhoun, Sec'y of War: Under the Command of Major Stephen H. Long, from the Notes of Major Long, Mr. T. Say, and Other Gentlemen of the Exploring Party, two volumes, Philadelphia, 1823. Reprinted in book form by University Microfilms, Inc., Ann Arbor. Say's description of the swift fox is given in volume 1, p. 486.