Board Thread:Lore Discussion/@comment-25267426-20150814222841/@comment-29832152-20170521065436

As I recall Arngeir says that there have been several Dragonborn across the ages, so there is the potential for a few to have crossed the Whalebone Bridge. Dragonborn aren't unique occurences like the Nerevarine, just rare. There probably hasn't been one for several centuries though, which is why Tsun sounds so happy if you tell him you're Dragonborn.

In answer to TS's race since I got curious, and the article  cites a Bethesda post that said that he was Atmoran.