The basic method for assigning scientific names that is now used for plants and animals originated with the Swedish naturalist, Carolus Linnaeus (1707-1778), and published in 1758. Known as the Linnean system of classification, it embodied a rational, orderly view of nature, and was one of the principal scientific contributions of the Age of Enlightenment to Western civilization. It organized flora and fauna into hierarchies, in descending order, of kingdoms, phyla, classes, orders, families, genera, and species. Linnaeus used Latin names because that was the international tongue of scholars in his day.
As Lewis and Clark observed and recorded Nature's progeny along the expedition's route, they did so with the most rigorous Enlightenment discipline they were capable of, although they had only a general knowledge of the Linnean system of classification. Based on the observable data–the bears' various colors being the most obvious–Lewis struggled with the problem of field-identification of a species that wasn't yet in the books.
Formal descriptions, classifications, and admittance into scientific literature, were the responsibilities of scholars back home, such as George Ord (1781-1866) ,the American naturalist who first used the captains' journals, and the specimens they collected, to organize the "discoveries" they had made.
Ord classified the captains' nemesis under the generic name, Ursus, which is Latin for "bear." He viewed it as a single, separate species, and gave it a designation that wrapped up the journalists' feelings in one emotion-packed word–"horribilis."
The grizzly that Lewis and Clark saw is now considered to belong to the same species as the Alaskan brown bear and the Kodiak bear, and is designated as the species arctos–the Greek word for "bear"–and sub-species horribilis. Thus is this group of bears, Ursus arctos, distinguished from the more numerous and less intimidating Ursus Americanus–the black bear. (The latter, not the grizzly, is the model for "Smokey Bear," and the "Teddy Bear.")
So today, perhaps merely out of respect for historical tradition, horribilis still identifies the sub-species found in Montana, Wyoming, Idaho, Washington, Alberta, British Columbia, the Northwest and Yukon Territories, and interior Alaska.