Dragonborn being formed seems to be a somewhat random occurrence and there must have been some of them in the past who weren't Nords. What might they have been regarded as in other cultures? They probably weren't called Dragonborn, I assume.

Or would Dragonborn only exist in the Nordic culture as it is a cultural hero like the HoonDing? That doesn't seem right if they were known about over in Akavir.

Reman Cyrodiil, who was a Cyrod in the most literal fashion humanly possible, was a Dragonborn. Most of the Septim line, too, who were most likely a wide mix of different Human flavours as the dynasty got older. Septim himself was probably primarily Breton, though possibly also somewhat Nord, or maybe it was only Hjalti who was Breton and Tiber himself was Colovian or Nordic.

Trying to tie down Tiber's ethnicity is kinda hard, tbh. Still, there have been Dragonborn of other races, and they would probably be referred to as such. Dovahkiin, on the other hand, is probably a specific Nordic term.

I know about the Septims, but they are kind of a different case as they get their dragon blood due to Alessia creating Akatosh and making a covenant with him, so the concept of Dragonborn to the Cyrodiils is still technically Nordic as the concept was taken from the Nordic culture and assimilated into the Imperial culture along with some concepts from the elvish culture in order to make everyone happy.