Pegasus
Reference: Pegasus
A horse with the wings of a bird attached. The composition is exact, and so is the violation: six limbs.
Scored on every axis
The clean case
Pegasus is the most straightforward entry in this group because nothing else is altered. It is a horse — four legs, hooves, equine head and lateral prey vision — with feathered wings added at the shoulder.
That is precisely the arrangement the four-limb constraint forbids. A real flying horse-sized vertebrate would have to give up its forelegs, as bats and birds did, and would land on two legs.
There is a second problem the anatomy raises, independent of limb count: mass. Flight muscle in birds runs to a fifth or more of body weight and anchors to a deep keeled sternum. A horse has neither the keel nor the power-to-weight ratio, and the wingspan required scales past what a shoulder joint could carry.