Dolphin

Note that this does not exist in Minecraft; it was written purely as a fiction aritcle.

Dolphins are friendly passive mobs that live in deep bodies of water. They are a light blue with a white belly and a large tail fin.

Behavior
If one encounters a Dolphin, the marine mammal will make clicking and whistling noises as a "hello". They are usually found in group and will attack sharks. If you hit them, they will tweet in a higher-pitched note and swim away. If there is a group nearby, they will also swim away, sort of reverse behavior when compared to wolves and zombie pigmen.

Habitat
Dolphins are found in deep bodies of water, otherwise sea biomes. It must be Y=65 or less in a volume of water in order to find one. Beware, this is also where Sharks are found.