Was Glorfindel's horse one of the Mearas?

[Zigûr]

Originally Posted by Inziladun

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.

Yes the wording suggests to me that it was innately a unique quality of Shadowfax that allowed him to stand before the Lord of the Nazgûl.
"Shadowfax who alone among the free horses of the earth endured the terror, unmoving, steadfast"

Originally Posted by Inziladun

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.

Yes; after all, the One Ring afforded Bilbo longevity when he was in his early 100s in the Shire, well before he moved to Rivendell.