At some point, somehow, the Assassin Michel Reuge found the aegis near Bath in England and later stored it in an underground a vault in London. In 1868, Evie and Jacob Frye gained access into Reuge's vault and Evie claimed the outfit for her own usage.
Behind the scenes
In Assassin's Creed: Syndicate, when the Aegis is worn, a visible shield of surrounded Evie Frye.
In Greco-Roman mythology, the Aegis is sometimes depicted as a shield with the image of Medusa's head emblazoned on it. Other sources state that it was a cuirass. What it was made of, or how and by whom, is debated. The alternatives includ the hide of baby Zeus' wet-nurse goat Amalthea, by Hephaistos and the Cyclopes, and even that it was the skin of a slain Gorgon.