Mr Tickle

Mr Tickle was a fictional character with his own series of books.

Other realities
A version of Mr Tickle existed as a real individual in the Daft Dimension. The Twelfth Doctor explained to Clara Oswald that they couldn't go to meet him as he "did not exist", unaware that Mr Tickle was reaching out for him with his long arms. (COMIC: The Daft Dimension 479)

Behind the scenes
Mr. Tickle was the titular protagonist in the very first Mr. Men book by Roger Hargreaves, published in 1971.

The inspiration for this character came from a question by Hargreave's 8-year-old son, Adam Hargreaves, who himself would go on to write all fourteen books for Dr. Men, a Doctor Who spin on the iconic children's series, beginning in 2017.