Newt Scamander wrote Fantastic Beasts and Where to Find Them in J.K. Rowling’s wizarding world.
The book is presented as a Hogwarts textbook by Newt Scamander, whose full name is Newton Artemis Fido Scamander. Harry, Ron, and Hermione’s school copy is described as having been written by Newt, with humorous notes added in the margins. The fictional textbook first appeared as a real charity companion book in 2001.
Rowling wrote that edition under the name Newt Scamander, with proceeds supporting Comic Relief and other charitable work. Newt later became the central character in the Fantastic Beasts film series, beginning with the 2016 film, in which he is played by Eddie Redmayne. Readers sometimes identify the real-world author as Rowling, which is correct for the published companion book, but the question asks for the in-universe author credited inside the wizarding world.