A banner... It would often bear his coat-of-arms or heraldry so that friend and foe could identify him in battle
A knight's personal flag is called a heraldic banner or a personal coat of arms. It typically features a unique design that represents the knight's family lineage, achievements, and values.
banner
pennon
A group of knights is called a "brotherhood" or an "order."
Knights of the round table.
It's on the tip left page above the flags of the man pouring a bucket of water on a the fire flag.
THE FIVE WOUNDS OF CHRIST. WORN BY THE KNIGHTS TEMPLAR.
The California flag is called the bear flag.
The Royal Standard is the monarch's personal flag.
The knights of white Kamelia flag has 5 red squares and 4 white squares
Pieces of land given to knights as payment were called fiefs.
Some knights were called sirs as they were knighted by a king/Queen for different reasons.
It is called "Red Ensign". The Ontario's flag is called "Red Ensign".