Wristwatch Shield

The Wristwatch Shield is a gift given to Percy from his Cyclops little half-brother, Tyson.

Percy's Shield
It is a shield that is encased within a watch. It has a series of engravings on it depicting the events that took place during the The Sea of Monsters. It saves Percy's life many times, and gets damaged once by the Manticore. Percy loses it forever when he throws it at Kampê to keep her away from Tyson long enough for them to escape.

Magical properties
It can transform into a watch. Since it was one of Tyson's first projects, and wasn't when he was in Poesidon's forges, it most likely was just a normal shield that could transform into a watch.