ROK
From Tolkien Gateway
ROK/ROKKO is an Elvish Root related to horses.
Unrelated Mannish roots of the same meaning were LUH and KARAB
Other versions of the legendarium
Tolkien experimented with the concept that ROK was a word related to fear and subsequently to Orcs. The origin of the root ROK as 'fear' was the horseman emissary of Melkor who abducted Elves from Cuiviénen. However later Orcs and fear, were related to a different root, ÓROK and RUKU.