Although the mysterious Musti weaves his way enticingly through J.R.R. Tolkien's The Story of Kullervo, the black hound appears only sparingly in the original Kalevala. Clearly, Musti and all his potential sparked Tolkien's imagination, for Tolkien makes the dog a character in his own right and, in doing so, enhances the plot and characterization of the original tale.
In the Kalevala, Musti turns up only at the very end of the story. Kullervo has just come home from taking his revenge on Untamo, and he finds an empty house. His parents and siblings have all died in his absence, but his mother has one last message for him from beyond the grave. She tells him to take the black dog Musti as a companion and go into the forest to seek the help and favor of the Blue Maids (Runo XXXVI). Kullervo and the dog enter the forest, but there Musti's role ends. Readers hear no more of him as Kullervo falls on his sword and ends his life although presumably Musti witnesses his death.
Tolkien, in contrast, makes Musti a character in his own right, developing him into a magical, mysterious hound who serves not only as a companion but also a guardian and teacher to Kullervo and his sister Wanona. In The Story of Kullervo, Musti makes his first appearance as an ally to Kalervo in his feud with Untamo. The dog devours the cattle Untamo sets loose in Kalervo's fields (6). The hound, however, is not present when Untamo raids Kalervo's homestead, kills Kalervo, and carries off the captives (8). Musti follows his family and later befriends Kalervo's children, teaching them about their father and about “things darker and dimmer and farther back even perhaps before their magic days...” (10).
Here Tolkien drops a few tantalizing hints about Musti's true nature. Musti, he explains, is “the wisest of hounds” (10). No one knows exactly where he came from or when he was born, but he is a powerful dog “of fell might and strength and of great knowledge” (10). He understands the secrets of wild creatures, but even more extraordinary is Musti's magic. He can shift his shape to take on the image of wolves or bears or cattle and perform “much other magic besides,” as he would soon demonstrate (10). After warning Kullervo of Untamo's violent designs, Musti gives the young man three of his black hairs, which protect Kullervo from death by drowning, fire, and hanging (11).
Musti remains a powerful companion to Kullervo even after the young man is sold into slavery. The dog follows him and wanders with him in the wilderness, teaching him to interact with wild animals (20). After Kullervo uses that knowledge, with a bit of magic of his own, to kill Āsemo's wife, he flees with Musti by his side (32). The hound's magic wraps around Kullervo, protecting him from harm and preventing anyone from following him.
The black dog disappears from the tale as Kullervo meets and enters into an incestuous relationship with his sister. Was Musti present during this time, or was he off on errands of his own? Tolkien leaves readers to draw their own conclusions. Musti returns only in the unfinished notes at the end of the story when Tolkien mentions that Kullervo trips over the dead body of Musti after the raid on Untamo (40). The magical dog has finally met his end, and readers wonder if he had been killed by the one to whom he had been such a loyal companion.