Deep Salamander

Deep Salamanders would be hostile mobs with 30 HP, and 4 attack damage on normal.

They would spawn below level 0 in water with a light level of 0. They would be about 2 blocks long (plus a tail), 2/3 block high, and 1 block wide (plus legs). They would be able to breathe in both air and water.

Their skin would be dark grayish (to blend in with deepslate) and they would have small bio-luminecent spots along their sides. When fighting, these spots would go dark.

The purpose of these would be to add some extra spice to swimming in deep caves or even the deepest deep oceans (See this post for my idea on very deep oceans).

When a deep salamander sighted an enemy, it would go dark and attempt to sneak up slowly before dashing the last few blocks, attacking once, and then running away a few blocks. If a player looked at a sneaking deep salamander, it would immediately dash in and attack. With proper application, this sort of behavior could make the deep salamander hide in holes out of sight and ambush unsuspecting players just to disappear again.