Rounding off the 2nd wave of Play Arts Kai Final Fantasy 7: Advent Children here's a review of one of the most underrated, and least favorite characters in Final Fantasy 7. To be honest, i know very little about RED XIII or Nanaki, his proper name. I believe though that it was intended by the developers since it was never mentioned which race he came from though he's a denizen and Guardian of the Cosmo Canyon, same as his father before him (Seto), and was raised by his adopted "Grandfather" Bugenhagen.
The name RED XIII, if i remember correctly, is a specimen signature assigned to Nanaki by Hojo while he's under incarceration and experimentation. It is not clear what kind of human/animal hybrid he is, a lion perhaps or a wolf since he howls. It is clear though that his intellect is similar to that of a teenage human though his lifespan is exceptionally long, as shown in the ending of Final Fantasy 7 when he visited the ruins of Midgar with his two cubs and his conversation with the immortal Vincent.