Whale
Reference: Cetacea
A land mammal that went back to the sea and kept its lungs. It still carries the pelvis of a walking animal, detached and buried in the body wall.
Scored on every axis
The leftover hip
Whales evolved from four-legged land mammals, and the record of that is still in the body: a reduced pelvis, connected to no spine and supporting no leg. Occasional individuals are born with small external hind limbs.
This is the shared-error pattern in physical form. A design starting fresh for open water has no reason to include a disconnected hip bone. An inherited body plan cannot easily discard one.
Breathing the hard way
Returning to water did not restore gills. Whales surface to breathe through a blowhole — nostrils migrated to the top of the head — and they exchange a far higher fraction of lung volume per breath than we do, store oxygen in muscle rather than lungs, and shut down peripheral circulation while diving.
Gills were never re-invented, which is a point worth holding when reading the mermaid entry.