Archaic form of the name Roheryn, a combination of roch “horse” and the lenited form of archaic †heruin “lady” (PE17/97-8).
Conceptual Development: In Words, Phrases and Passages from the Lord of the Rings from the late 1950s or early 1960s, Tolkien spent some time analyzing the origin of this name. At first, he considered having the suffix -uin be derived from an archaic genitive construction based on the primitive feminine form ✶kherī (> Q. heri “lady”) + ✶ān(a), but he abandoned this idea, deciding instead that heruin developed directly from a variant primitive form ✶kherūnī (PE17/97-8).
prop. n. 'Steed of the Lady', Aragorn's steed given him by Arwen. Pure S. form, also Rocheryn < Roch(ch)eruin, softened Gondor form Roheryn. >> Rocheruin, Rocheryn, Roheryn