Tirannûnmir: King's Companion (LoTR Fanfic) Book 1

Tirannûnmir, the Elves call me. Wolf is what the Bree folk call me. However, the name my father gave me is Meltir and I am Aragorn's Companion.

Meltir is of the Dunéhuain, "Hounds of Westernesse" in the common tongue. Similar to the Beornings but gifted with a slightly deeper understanding of magic by the Valar and to take the form of large wolf-like hounds. Her father's race came with the Numenoreans across the waters and eventually sworn an oath to protect the line of Kings. Many of them died in the war against Morgoth and even more in the first war against Sauron doing their duty alongside their Dunédain companions. Meltir's ancestor died avenging Isildur by killing the orcs that had shot his companion. And so, the Dunahuan was bound to protect the last heir with her life and restore him to his rightful place from the moment he was born. When Arathorn and Belamrûn fell, Elladon and Elrohir took Gilraen, her son, and Meltir to the Last Homely House of Lord Elrond, who fostered the two children, naming them "Estel" and "Tirannûnmir".

Please note that this is lengthy and follows the books, not the movies. Thank you!