Arthur Conan Doyle

Sir Arthur Conan Doyle was a writer most well-known for the books about the detective Sherlock Holmes. Doyle fictionalized the adventures of Sherlock Holmes and Dr. John Watson. (NA: All-Consuming Fire)

In 1880, Doyle aided the Doctor and Sarah Jane Smith, though the Doctor suspected that they may have met before (MA: Evolution). In the early 1900's, he met the Doctor met and aided the Doctor again. (TDA: Revenge of the Judoon). Some years later, he examined the famous Cottingley fairy photos and pronounced them genuine. (TW: Small Worlds)

Minor references
As a doctor, he accompanied Redvers Fenn-Cooper along on an expedition to observe giant reptiles, which resembled dinosaurs, though at first he had trouble believing they exist.. (DW: Ghost Light)

Behind the Scenes
Though Jack Harkness implies in Small Worlds that such a hard-minded skeptic would have trouble accepting the Cottingley fairy photos, in reality, in his later years he accepted the existence of the supernatural, including the literal existence of fairies.