Dinosaurs are animals that live in the lost cities. The elves have an initiative to not let any animal go extinct, and dinosaurs are not an exception. In the book, it is shown that dinosaurs never went extinct. The elves prevented them from extinction by keeping them in preserves, in order to hide them from humans.
There is a tyrannosaurus rex living in the Havenfield reserve named Verdi. Verdi is described as having "huge yellow eyes, sharp claws, and a pointed snout," as well as "sharp fangs," and "neon feathers". Verdi is also described as being "twice the size of an elephant." ( Details on Verdi are in Book 1, page 202, e-book version)
There are also an undisclosed number of dinosaurs living in the mountain reserve.