A mythological and aquatic mob

My idea is to add a mythological and aquatic mob to the game, this would be found in the depths of the sea and when defeated they would release you diamonds, gold, iron and a food that is eaten in parts (like cake) and each bite will fill you with all the hunger bar (something just like for a mythological mob) this is not aggressive but how much you attack it will take a lot of life and so it would be good to attack it with armor. You would have 0.05% that you find it just by chance, that's why I think that in the ships there are maps that guide you to this mythological mob, this because the pirates liked diamonds and ect, then when they realized this mob they started to follow him and they knew his location thanks to the testimonies of the people who found him by chance.