I would imagine that Imu has either encountered a DF user or a mythical Nidhoggr. That would make the most sense, but the story hasn't guaranteed this.
I think Nidhoggr is by far more obscure than Nika and less relevant to the mythos, but closer to Nika than Buddha or Seiryu, who have not needed any explanation within the narrative. If Imu learned through some sort of literature then I guess it would have to be introduced, since we don't know that this is something Imu would be able to derive from something like a poneglyph