A mouth organ, also known as a harmonica, is a small, handheld Muggle wind instrument. Two distinct mouth organs appear in the series. The first is the personal instrument of Rubeus Hagrid, which is described as a “battered old mouth organ.” This suggests it is well-used and has been in his possession for some time. The second is a “brand-new” mouth organ that Rubeus Hagrid gives to Harry Potter as a Christmas present during his first year at Hogwarts.
The mouth organ possesses no known magical properties and functions identically to its Muggle counterpart. Its primary use in the story is as a musical instrument for Rubeus Hagrid to express his emotions. When upset after Buckbeak the Hippogriff injures Draco Malfoy, Hagrid pulls out his battered mouth organ and plays a mournful tune. It is a simple, non-magical way for him to cope with his distress. While Harry Potter receives one as a gift, he is never depicted playing it.
The mouth organ appears on two notable occasions in the narrative, associated with Rubeus Hagrid.
The mouth organ is a minor object that serves primarily to develop the character of Rubeus Hagrid. Its presence underscores his simple, rustic nature and his reliance on non-magical comforts. It acts as an outlet for his powerful emotions, particularly sadness and frustration, which he often struggles to express in words. Hagrid's gift of a mouth organ to Harry is also significant, as it represents a thoughtful, personal present from a caring figure, reinforcing their bond and contributing to Harry's first experience of a loving, familial Christmas.
There is no known information from J.K. Rowling or other official supplementary materials regarding the deeper significance or origin of the mouth organ in the series. It appears in the film adaptations in scenes consistent with the novels.