A monster slain by Arthur in Le Chevalier du Papegau.
It lived in the sea and its armor, helmet, shield, horse, and sword were all part of its natural body. It had been terrorizing the fairy Lady of the Blonde Hair of the Amorous City, and she summoned Arthur’s help. A violent sea storm followed the Fish-Knight’s death.
Source
Le Chevalier du Papegau | Late 14th century or early 15th century