I don't know if this is answerable under forum rules, particularly:

As detailed in our rules, answers should be in-depth, comprehensive, accurate, and based off of good quality sources.

This isn't a complaint about the rules, but about the situation involving Sponsian. The plain fact is, I know of no source whatsoever that mentions him. And I've spent some time digging.

If the coins are accurate, it places Sponsian in Dacia during a period of invasions and pandemic. Rome wasn't able to project much power there, and Aurelian would eventually end up completely evacuating and abandoning the province.

It could be Sponsian was a general who was elevated by his troops, as happened with Postumus in Gaul. It could be he was a regional big-shot of some sort who proclaimed himself.

But as far as I can tell, all we have is speculation. I would absolutely welcome any source if someone knows of one. As an avid reader on 3rd Century history, I am fascinated by this.