Was Glorfindel's horse one of the Mearas?

[The Mouth of Sauron]

In LOTR Glorfindel's mount Asfaloth is described as an 'Elf-horse'. Is Asfaloth faster than Shadowfax, being an 'Elf-horse', or one of the Mearas subordinate to Shadowfax?

[Inziladun]

I don't think Asfaloth was anything but a normal horse. Gandalf told the Council of Elrond he had "never seen the like of Shadowfax" before, which tells me that horse was unique.

[Zigûr]

Asfaloth was also terrified of the Ringwraiths, according to Frodo, while Shadowfax wasn't - although that seems to have been unique to him alone even among the Mearas. That being said, Shadowfax was unique in holding his ground before the Lord of the Nazgûl specifically, so it's possible that other Mearas would have at least endured before the lesser Ringwraiths.

Perhaps an "elf-horse" is a term applicable to such steeds as were bred and husbanded by Elves because they might have perhaps had a little more of the "spiritual" to them than their mundane counterparts, largely through being tended by the higher arts of Elves, giving them a strong connection with their riders as well as speed, grace and vigour. This, however, would presumably not be equivalent to the physical and spiritual fortitude of those horses descended from the "high" stock of Oromë, who presumably benefited from a much more substantial Valar-derived blessing.

[Leaf]

I think Zigûr is right. There seems to be a special bond between "Elf-horses" and their riders. The communication and empathy between the horse and the rider could be the main difference which seperates Elf-horse from normal animals. Another good example for this would be Bill, Sam's Pony:

Originally Posted by J.R.R. Tolkien, The Fellowship of the Ring, The Ring goes south

The stay in Rivendell had worked a great wonder of change on him: he was glossy and seemed to have the vigour of youth. It was Sam who had insisted on choosing him, declaring that Bill (as he called him) would pine, if he did not come.‘That animal can nearly talk,’ he said, ‘and would talk, if he stayed here much longer.’ [...]

So it seems to me that Bill the Pony could have become the first case of an "Elf-pony", if it had stayed in Rivendell for long enough.

[Gorthaur the Cruel]

It may be that the reason Shadowfax was able to endure the Witch-king was because his rider was none other than the bearer of Narya, the ring of fire that kindled hearts, rather than his own breed. In the case, of Bill the pony, he was also under rejuvenating effects of Vilya, the ring of air, used to preserve and maintain Rivendell. Why else was Bilbo ale to age still so slowly even bereft of the One? It's because he also dwelt under the influence of Vilya in Rivendell, for a time.

[Inziladun]

Originally Posted by Gorthaur the Cruel

It may be that the reason Shadowfax was able to endure the Witch-king was because his rider was none other than the bearer of Narya, the ring of fire that kindled hearts, rather than his own breed.

It's an intriguing idea. I'm inclined to think the courage of Shadowfax was due to his mearas status, with perhaps Gandalf being the White providing an enhancement. No doubt Narya would have been a help as well.

Originally Posted by Gorthaur the Cruel

In the case, of Bill the pony, he was also under rejuvenating effects of Vilya, the ring of air, used to preserve and maintain Rivendell. Why else was Bilbo ale to age still so slowly even bereft of the One? It's because he also dwelt under the influence of Vilya in Rivendell, for a time.

I have to disagree here. I think the 'mark' left by the Ring was what sustained Bilbo. It certainly did the trick for Gollum, even though he too was bereft of physical possession of the One.