Why does the scientific name of the grizzly bear contain the word "horribilis"?

Both "ursus" and "arctos" mean "bear"--"ursus" in Latin, "arctos" in Greek. And yes, "horribilis" is the Latin root of the English "horrible." "Ursus arctos" is simply the brown bear, which is found in North America, Asia, and Europe--the double name may imply that it, of all bears, is the true and archetypal bear. (Similarly, the scientific name for the horse, "equus caballus," uses two words meaning the same animal, in this case the first from classical Latin and the second from late or Vulgar Latin. The donkey, being a related and horselike animal, is designated "equus asinus.") "Grizzly," spelled this way, means grey or grizzled and refers to the particular strain of u. a. with that coloring, but the word "horribilis" may suggest an association with "grisly" as well.
Ursus is Latin for bear
Arctos is Greek for... bear
Horribilis is Latin for horrible (or 'horrible' is the English derivative of the older Horribilis)
Ursus Arctos is the Brown Bear
Ursus Arctos Horribilis is the Horrible Brown Bear, or Grizzly Bear
Source(s):
https://en.wikipedia.org/wiki/Grizzly_bear
https://en.wikipedia.org/wiki/Grizzly_bear#Meaning_of_"grizzly"

Naturalist George Ord misinterpreted the word "grizzly" which meant the grizzled look of the hair of the species. He thought it meant "grisly". He used the latter word to form part of the latin name "Ursus arctos horribilis".

https://en.wikipedia.org/wiki/Grizzly_bear

http://seapics.com/feature-subject/bears/grizzly-bear-pictures-001.html

Horribilis designates the grizzly bear as a sub-species of brown bear. Ursus arctos refers generically to any brown bear.
https://denverzoo.org/animals/grizzly-bear/

Per this next link, the Latin horribilis did not mean horrible, but dreadful was closer to the actual meaning.
http://www.nytimes.com/2005/01/23/magazine/23ONLANGUAGE.html

So, the classification of a grizzly bear describes a brown bear that can scare the bejesus out of you. Imagine a 9-ft tall, 800 pound meat-eater chasing after you at 30 mph...