Dragons, in the original sense of the word, are real animals. These iconic monsters of European folklore are the literary descendants of ordinary snakes that evolved through the centuries with much help from the discipline of natural history. Classical authors applied the term dragon to large snakes such as Aesculapian snakes and pythons. Over time, so many fabulous traits accrued in the descriptions of these animals that by the Renaissance dragon descriptions strained credulity, and eighteenth-century scientists dismissed dragons as mythical. Particularly important among dragon descriptions in the literature of natural history is that of Conrad Gessner in the snake volume of his animal encyclopedia Historiae animalium. Published in 1587, it incorporates a more comprehensive review of dragon lore and literature than any previous work. This makes it an important reference for describing the conceptual evolution of the dragon from an ordinary snake into a fabulous monster. The volume was first published in Latin, then in German in 1589 under the title Schlangenbuch. Here we use Schlangenbuch’s sources to trace the evolution of the dragon in the literature of natural history, with comments on the dragons of folktales, myths, and legends. We also present the first English translation of the dragon section of Schlangenbuch, annotated to identify Gessner’s sources and their contributions to the conceptual evolution of the dragon. In addition we present, similarly annotated, the first English translation of the dragon section of Ibn Sīnā’s Canon of Medicine, a source that Gessner repeatedly cited.