[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.