It is a curious fact of the history of Middle-earth that Maglor Feanorion was the premier horse-master of Beleriand. Between the hardy native breeds and the powerful Valinorean race - the ultimate ancestors of the mearas - there was no contest, for even setting aside the effect of the Two Trees, Valinor was a land of wide plains, while Beleriand was heavily forested.
But the horses of Aman never crossed the Grinding Ice; the only Valinorean steeds to reach Middle-earth were those carried by Feanor's host in his ships. Given the tense relations between House Feanor and the rest of Beleriand, it is clear that the Feanorian horses would have remained just that.
But Maedhros lived on a mountain; Caranthir, Celegorm, and Curifin guarded hilly regions and mountain passes; Amrod and Amras hunted in the ancient forest. Of Feanor's seven sons, only Maglor lived in horse country - in fact, in the greatest horse county in Middle-earth. For Maglor's Gap was the eastern gateway between forested Beleriand, and Ard-galen - the massive plain between Dorthonion and Thangorodrim. Until the Sudden Flame, it was a sea of grass - and of Noldorin warriors, and of their horses.
So Maglor would have raised the greatest horses in Middle-earth; tall and proud, fleet of foot, heirs to the horses of Valinor, and even to Nahar himself. And they would have been long-lived - the one Valinorean animal we know the lifespan of is Huan, and he lived at least 460 years. Maglor would have had a lot of time to get to know his charges.
So what would he have named them? Given Maglor's somewhat tense relationship with his family, the typical Feanorian names would have been out. But the first thing everyone knows about Maglor is that he was a harper - and we all betray ourselves in the metaphors we use. Maglor would have seen his horses as allegro, as adagio, as prestissimo, their personalities likening themselves to music.
We do not have a Quenya (for Maglor of House Feanor would certainly have used Quenya) musical vocabulary comparable to our Italian one. But different instruments are suited to different tempos and styles of music, and so a slow horse might become NainiŽ, 'Lament', and a frisky, ever-juvenile steed might be NyellŽ, 'Bell'. A steady walker could be known as LairŽ, 'Poem', and one who could hold a rhythmic trot as LŪrŽ, 'Song'.
And Maglor's personal mount? We each seek out personalities like to our own - and so what else could his horse be known as but Nandaro - 'Harper'?