New entity type

i dont know exactly yet, but i have a good idea. underground water creature, probably makes it spawn underground, in water. water creature makes it spawn in water, water ambient maybe spawn in water, but only in like the dark??? misc, i have no idea, maybe it just, doesn't spawn or has to be spawned through commands, i first thought it meant it had no condition and spawned anywhere you wanted without any limitations. sorry for not helping much

Ambient water spawn at anytime in the water, cuz ambient is at anytime, creature water is the day and under ground is in cave i guess, i searching for misc too