Midgardsormr is definitely not a primal. He's the Father of Dragons - the first of their kind and the King of Kings.
How do we know for sure? Did Fernehalwes actually say so? Did Fernehalwes quote Yoshi-P actually saying so? Did Corvinoobus find out that the Waning of the Sixth Sun BluRay bonus disc calls Midgardsormr an "elder primal" and explicitly let everyone know in no uncertain terms that he "ain't no primal"?
The answer is D) All of the above.
Incidentally, Midgardsormr might not be from "space". It's worded pretty ambiguously. He "came to" this planet in "a time before time", yet we have almost zero knowledge of the span between when Light and Darkness were one and when corporeal life arose, and life is usually credited to Hydaelyn. It could just as easily be that he just kind of came into being with the corporeal world. That might be why his name means world serpent. If splitting the original status quo into a planet and moon stressed the boundaries of the realm so hard that it fractured into fourteen facets, what's beyond? Granted, it'd be pretty funny to see a map of the known universe with one planet, one moon, a sun, and the rest is just "There be dragons."