The implication there of course would be that Feros is the Leviathans' homeworld, presumably with it's oceans long since dried up. That or Thorian spores were somehow transported there at some point in the past...perhaps as a handy food source?
Well I read on one of the TV tropes fridge pages for the Mass Effect games that there was a race mentioned in one of the Mass Effect games (I think it was ME 2) that was apparently clashing with one of the dominate races of a cycle who's name is similar to the Thorian that led to speculation that the Thorian used to have its own little interstellar empire. So it might not be native to Feros.
Didn't Exogeni give it that name? If so then it's probably not it's true name anymore than "Prothean" or "Leviathan" are the *actual* names of those races. It's probably just a co-incidence. More to the point, the Thor'han was a spacefaring race. If they were the same species as the Thorian, what are the odds it could have gone unnoticed for two consecutive reaping cycles? We know the Protheans on Feros encountered it, just as we can be sure the Reapers cleared the planet out. So the reapers must have known of it through them and the fact that it still exists means that the catalyst didn't regard that species as significant, otherwise they'd have glassed the planet instead of just stripping it.
Not that it means the Thorian must be native to Feros either. Like I said, some other race could have transported it or it's spores there in some previous cycle. There's no way to know for sure.
Plus, contrary to what whoever posted that idea on TVTropes said, as far as I can tell there's no confirmed instance of the Isunannon and the Thorian coming into contact. All we know is that the Thorian was on Feros long before the Protheans. If Feros is indeed it's homeworld then fine, it was there when the Isunannon were on the Citadel, however, while it's certainly a possibility, we don't know for sure that they ever went to Feros or encountered the creature there. For all we know, their survey team arrived when the Thorian was hibernating, decided the planet wasn't a viable colonsation prospect and never came back.