A Mastodon is an elephant-like creature believed by humans to be extinct. They are similar to woolly mammoths, but smaller and with sharper tusks. The Mastodon is the Level Three mascot at Foxfire, and Sophie had to dance in the Mastodon costume at the Opening Ceremonies. Edaline and Grady have several Mastodons at Havenfield, and according to Edaline, a Gorgonops once ate a Mastodon at the estate. It was also seen in the first book that they are capable of emitting an earthshaking squeal that made Sophie's ears ring. This occurred after Edaline mounted one of the creatures.
- Keeper of the Lost Cities is the only book in the Keeper of the Lost Cities series where the species is called "Mastodons". In every other book, they are spelled "Mastadons", regardless of the official scientific spelling of the word. Readers think this is a mistake of Shannon's, like Logner-Heks.
“As an Empath, Keefe could feel the terror coursing through her veins like a herd of stampeding mastodons.”