In Greek mythology, the aegis was a type of shield or breastplate. The aegis was associated primarily with two gods - Zeus and Athena. Zeus had a goatskin aegis, while Athena adorned the aegis with the head of a Gorgon.
According to the Iliad of Homer, the aegis was made by Hephaistos. Here is the relevant passage from the epic:
“The smith Hephaistos gave this shield to Zeus
to carry and strike fear in men.”
(Homer, Iliad, Book Fifteen, lines 307-8)
The ancient Greek idea, which can be roughly paraphrased to “helping friends and harming enemies”, applies to the aegis as it appears in the Homeric epic. There is a scene in which Zeus speaks to Apollo:
take for yourself my tasseled shield of stormcloud,
and shake it hard with lightning overhead
to rout the Akhaian soldiers. God of archery,
make Hektor your own special charge.
Arouse his utmost valor till, in rout,
the Akhaians reach the ships and Helle’s waters.”
(Homer, Iliad, Book Fifteen, lines 229 ff.)