Etymology of Osettic "Nart"? (the suffix?)

I've been looking into the etymology of the Ossetic name "Nart". The connection with PIE
*(a)ner- "vital energy; man" seems straightforward enough, though I don't yet understand
the -t. I thought perhaps some kind of proto-Iranian plural/collective -ta suffix might be
involved, though checking Pokorny, I see he suggested an earlier Iranian form like *nar-Ï`ra-
(I think the symbol before the -ra- is a theta/θ). I'm not sure what a "-thra-" suffix might
imply. Any thoughts?

Cheers,
Carl