There are multiple characters with this similar name. You may be looking for Old Man Alligator.
The Old One is a red fire dragon described as a "vast, primordial power. An ancient intelligence birthed when the world was still young." After his mate was killed by humans, he retired inside a volcano to live out the rest of his days alone. In Dark Storm, Dax and the Old One become one to fight the vampire Mitro: "Not two beings merged together, but two souls connected by a single body."