Balios is a black horse owned by the Shadowhunters of the London Institute in the 1870s, cared for by Thomas Tanner, and later his brother, Cyril. Along with Xanthos, both horses were named by Will Herondale after the immortal horses of Achilles. Both are said to be magical horses; stronger and faster than ordinary ones. Out of the two, Balios was Will's favorite.