Harrison G. Manxmouse began existence as a unique ceramic figurine, formed unintentionally by a drunken potter in Buntingdowndale who aimed for a mouse. Instead, he emerged with a blue body, rabbit-like ears featuring orange interiors, kangaroo-like hind legs, monkey-like front paws, and no tail. He spontaneously burst to life during the night.

Born without natural birth, Manxmouse possessed inherent fearlessness. This defined his early encounters, like meeting a shapeshifting Clutterbumph (or Dororon/Gholis), which failed to frighten him by taking the shape of a non-existent fear. Throughout his travels, numerous characters warned him of a prophecy foretelling his doom: being eaten by the mythical Manx Cat.

A schoolgirl named Wendy H. Troy formally christened him Harrison G. Manxmouse during an adventure. His journey involved meetings with figures like the aviator Captain Hawk and the runaway circus tiger Burra Khan (or Bula Khan), consistently revealing his politeness, helpfulness, and bravery. These experiences, especially the repeated prophecy warnings, gradually introduced the concept of fear to him.

His development culminated in a deliberate journey to confront the Manx Cat on the Isle of Man. In the original narrative, he nullified a document called the Doom by demonstrating willingness to defend himself. The anime adaptation resolved differently: the Manx Cat, having grown fond of Manxmouse, chose not to consume him, defying the prophecy through mutual affection.

Following this resolution, Manxmouse established a peaceful life and eventually started a family with a field mouse, marking his transition from a solitary creation to an integrated member of a community, having overcome his foretold fate.