THALIA COME HEREEEEEEEE

• The X, although technically the shield of Zeus, was carried by Athena. 
• She let Y borrow it so he could slay Medusa, since looking at her directly would have turned him into stone. After he had returned to Seriphos and turned Polydectes into stone, Y placed the head of Medusa on the X and returned it to Athena.
• The shield was originally made by Hephaestus, using the head of Medusa as an imprint on the face after Y turned over Medusa's head to Athena
•  A replica of the original shield is owned by Thalia Grace in the Percy Jackson Series                                   
  

                                           

                      Answer (highlight to read) : 'X' - Aegis and 'Y' - Perseus



And again courtesies to B.K.J.Venkat Krishnan