J. K. Rowling has stated that she initially wished to include a dog-lover in the story to contrast with the cat-loving Arabella Figg. Mopsy would take Sirius Black into her home while he was on the run, thinking he was a stray dog. However, Rowling's editor asked her to cut Mopsy out of an early draft of Harry Potter and the Goblet of Fire. Rowling chose to have fugitive Sirius stay inside a cave instead, where he could freely discuss important matters with Harry, Ron, and Hermione without Mopsy's interference.
It is never explicitly stated that Mopsy the dog lover from J. K. Rowling's Official Site and Mopsy Fleabert the author of Animal Ghosts of Britain from Pottermore are the same individual; however, given the unique name with a tie to their similar interests, it is believed here these both refer to the same individual.