1. What mythical creature does Rick Riordan allude to in the city of Los Angeles?

Los Angeles in the gate of the Underworld in the Lightening Thief. As far as what mythological creature he alludes to, I will need you to provide a chapter number. If your question pertain to Chapter Seventeen, it might possibly be Nereid, "a spirit of the sea" or Cerberus, the hound of Hades.