the.com/cetacean
A mammal that quit land, kept the lungs, and never looked back.
means Any member of the group of fully aquatic marine mammals including whales, dolphins, and porpoises.
from From Latin cetus, sea monster, via Greek ketos — the same dread that named astronomy's monster constellation Cetus.
Land ancestorsDescended from hoofed mammals that returned to sea.
Conscious breathingThey breathe voluntarily, so sleep half a brain.
Hidden hipsTiny leftover pelvic bones float, attached to nothing.