Agnus Dei translates as the “Lamb of God,” and is also known as the Paschal Lamb. It is symbolized pictorially as a lamb with a halo, proudly trotting along, carrying a banner and a cross.
Lambs were commonly sacrificed during the time of the Passover, the blood sprinkled in the doorway or rubbed onto the lintel, so the connection was made because of the sacrifice of Christ.
Part of the Catholic mass includes the plea, repeated three times:
Lamb of God, who takes away the sins of the world, have mercy on us.