A Mastodon is a "wooly mammoth looking thing." Edaline and Grady have some Mastodons at Havenfield. The Mastodon is also the Level Three mascot at Foxfire, and Sophie had to do a dance in the wooly costume at the Opening Ceremonies. According to Edaline, a Gorgonops ate a Mastodon at Havenfield. It was also seen in the first book, that they are capable of emitting an earthshaking squeal that was capable of making Sophie's ears ring. This occurred after Edaline was boosted onto their back.
- Book 1: 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.