Well, the horse Nahar was without a doubt one of the most powerful and exceptional horses ever described in ME, but I cannot find any reference that this would be the ancestor of all Mearas, well at least not in those words.
Oromë and his steed were known to the horse-loving people of Rohan, who claimed that the Mearas had ancestors brought out of the west by Oromë himself.
So, maybe you could make the conclusion that Nahar was the ancestor, but
Quote:
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Felaróf was a very remarkable horse indeed. It is even recorded that he could understand the speech of Men. He gave rise in turn to a race of wonderful horses, the Mearas, who according to tradition could only be ridden by the Lords of the Mark, Eorl's descendants. It was from this line that Gandalf's horse Shadowfax came, making him a descendant of Felaróf himself.
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(The Encyclopedia of Arda)
Maybe there are other sources who can give some more information?