Was Glorfindel's horse one of the Mearas?

[Galin]

I agree that the passage from the late Glorfindel text I cited (different from the description in Appendix A) implies that it was Glorfindel's influence which helped his horse. That said, while it was noted of the Mearas: "Men said of them that Béma (whom the Eldar call Oromë) must have brought their sire from West over Sea.'"

... it's also described (the constructed Silmarillion), that the Elves had horses whose sires came from Valinor (I broke up the paragraph below for emphasis here, or for some reason anyway):

Fingolfin and Fingon his son held Hithlum, and the most part of Fingolfin's folk dwelt in Mithrim about the shores of the great lake; to Fingon was assigned Dor-lómin, that lay to the west of the Mountains of Mithrim. But their chief fortress was at Eithel Sirion in the east of Ered Wethrin, whence they kept watch upon Ard-galen; and their cavalry rode upon that plain even to the shadow of Thangorodrim, for from few their horses had increased swiftly, and the grass of Ard-galen was rich and green.

Of those horses many of the sires came from Valinor, and they were given to Fingolfin by Maedhros in atonement of his losses, for they had been carried by ship to Losgar.

In any case