From Tolkien Gateway

ROK is an Elvish root related to horses.[1]


Other versions of the legendarium[edit]

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.[source?]