Maybe, but unlikely, as Sythe was already hit by two fireballs, one after Nibs was, and is not smoldering and the wispy dark shadow make up for Nibs was already shown on a page where there was no fire.
It was suggested she could be a baby dragon, but that seems to have been more meant as a red herring, or an allusion that she wasn't an ordinary squirrel. We know she is a thing that cannot die, and I don't really think she's far enough away from the manor that your suggestion that she's some kind of byproduct of the experimentation couldn't come to pass (it would be kind of a nifty tie-in, really). We also have the extended word building where Tom has drawm other creatures likes Wisp, and in canon proof that ghosts exist, and Remus said this world has familiars, which can also be spirits taking animal form in some tellings. She could be some kind of spirit without it breaking world building or canon. She could be some sort of agent of the mask, maybe too? She's more than likely a supernatural creature since again, she respawns when her incarnation/vessel dies.