An amphithere is a dragon from some European mythologies is described as a feathered snake, long body and wings with feathers, so basically can be stretchily compared with some Quetzalcoatl representations.
So this is a proposal about how a creature like the described could exist and evolve.
A dromeaosaurid can supply the main features, a feathered reptile with a long tail, feathered wings, specially seems that has all the features, except one, doesn't having legs or hind limbs, a really important aesthetic point.
Nevertheless is needed a reason to explain the hindlimbs lost in a dromeosaurid, but keeping the wings.
There are other problems that this could cause like the locomotion because the dromaeosaurid tails weren't stronger as snakes or to walk with just the arms and the teropods arms are permanently pronated.
So, why a dromaeosaurid would evolve to lose its legs?
Edit: I just found a drawing which help to examplify this.