I'm leaning towards "Hammer that ate a Squirrel Fruit" rather than "Squirrel that ate a Hammer Fruit", myself.
If Ragnir has outlasted several Elbaph Kings, that means it's OLD. Giants can live more than 400 years, if Jorul and Jarul are any indication.. So, it's pretty likely this Hammer-Squirrel is over a 1000 years old. Either coming from the Void Century or even before that. Possibly coming from the First World.
That would be PRETTY long for a squirrel to live. Even a Elbaph squirrel with a longer than normal lifespan. Elbpah Giants live about 3 times longer than normal Humans. The average squirrel can live around 10 to 20 years in the real world. So, even at the high end of that, Ragnir should have died at age 60, if he was "just a normal squirrel". That kind of makes me think Ragnir could just be a Hammer that ate a Squirrel Fruit. Because then...Hammers don't really age. If Ragnir was a Hammer, he'd be functionally immortal. And again, it's possible this is a object that ate a Devil Fruit, because it's probably old enough to have been around since the Void Century or maybe even the First World. The technology would have been there, supposedly
https://en.wikipedia.org/wiki/Squirrel
I can't really disprove the idea that Ragnir could just be a "legendary" Squirrel that ate a Hammer Fruit, though. And we have seen some curiously long-lived animals. The Sea Kings seem to have been living since the Void Century, given their comments about "those two meeting again". As has Zunesha, since he was friends with Joyboy. Ragnir could be like them. Possibly with some tie to the Ancient Weapons or...something else. Possibly having curiously long lives just by absorbing bits of Joyboy or Nika's powers? Or just from Advanced Ancient Technology? So, if Ragnir turns out to have some connection to Uranos, then that could mean he's just a VERY long lived Squirrel, just like how Zunesha is a very long lived elephant.
Although, there is the possibility that Ragnir could just be...something else. Something we haven't quite seen before. Could Ragnir be similar to whatever the Gorosei are? Possibly not using a "Devil Fruit" per se, but some other power? If the Gorosei really aren't using Devil Fruits, and are instead "Yokai", then maybe Ragnir is something similar, just using Nika's power instead of Imu's?
Random thought time!
Could Ragnir actually be one of the original Nika's "weapons"? We already know from various depictions that Nika wielded a Sword, a Spear, and a Shield. Those could all be different "incarnations" of Nika, or they could all just be the same guy. Could Nika also have had a hammer? Could ALL of Nika's weapons actually be animal companions? Kind of like the legend of Momotaro? Or something like Tarzan or the Jungle Book? Could Nika have had a bunch of weapons that had various Zoan Fruits (or animals that had weapon Paramecia Fruits), literally making himself a small army of "friends" he could use as weapons? What little we've seen of Nika does kind of give me a "jungle boy" vibe.
So, I don't know. We really don't know enough to guess. I'm leaning towards "Hammer". But, there are various ways to make "Squirrel" happen. This is probably going to end up being a SBS question. Hopefully, Oda doesn't just sidestep the question with a joke. Or just say that "Ragnir thinks this is too personal of a question!".