15 to 20 feet, or 5 to 7 metres with horns and teeth is quite a stretch for a flying animal in Earth's gravity and atmospheric pressure, but it is not completely impossible.
The most capable flying creatures on Earth are around 1kg in weight at most. Larger fliers are less capable of rapid manouvres, and require a run-up in order to take off.
Supposing that we could have a vertebrate animals that, by some genetic happenstance, mutated to have an extra pair of functional limbs at some time in the distant past. Unlikely, but not impossible or beyond the bounds of belief. That at some later point, one set of limbs - probably the centre set - evolved into wings. At this point, we have a flying creature that has four legs and a pair of wings. It is probably quite small, no bigger than other early fliers, probably in the sub-1kg range.
At this stage, the flying critter needs to be a carnivore. It would hunt by flying around and swooping down upon its prey, grasping it with its front limbs and biting it, then chewing it up before swallowing it.
Once these critters have evolved to be good fliers, and various populations have diverged from one-another, one population might evolve to have mate selection criteria that favours horns and great length, displayed in flight, possibly with an offering of prey, possibly with a display of teeth. The thing with sexual selection is that it can justify the most outrageous adaptations, because if another critter has bigger and sexier traits than you, they get to breed and you don't.
So, we have sexual selection for horns, long, flying, predatory and teeth, for this creature. We also have the purely physical selection criteria that flying creatures must be light. These selection criteria would favour critters that meet all six of these criteria. As the mass of the critter goes up, we can retain all four legs, as they will be necessary to propel it along the ground in its take-off run-up, and they serve a purpose in prey capture.
Now... how to make it really big? Sexual selection will help, since in sexual selection, the gender that expends the most energy in reproduction, usually female, can be choosy and will select only the mate that best fits their selection criteria. Over the generations, these critters will stretch out, becoming bigger, but also longer and thinner.
So, what do we end up with? Our dragons will be 15 to 20 feet long, but they'll be thin like snakes, with torsos that are narrow from side to side, but thicker vertically, to allow for the bone and muscle necessary to support their bodies. The spine would be muscular, as this creature would likely bound like a mustelid when making its run-up ass a precursor to flight. The limbs would be short and muscular, with grasping claws, to retain functionality while minimising weight. The bones will be hollow and light like a bird's. They will be muscular around their torso where the wings connect, to power its flight. The neck will be long and serpentine, and the head will be light. The horns will look big, but will be hollow and very light, and probably a different colour that screams 'Look at these!' The teeth must be functional, as this critter is a predator, but in order to lighten the head, the head is likely to be tall and narrow, to allow the maximum necessary biting strength at the minimum weight. The snout would be short, more like a cat's, as this is likely a sight-hunter which has little need of a long nose to support a good sense of smell. The eyes would be large and forward-facing, befitting a predatory species.
The largest flying species on earth is thought to have been Quezalcoatlus northropii. It is debatable as to how much they may have weighed, but it is reasonable for this dragon to weigh in the range of 70-100 kg.
With this size and these adaptations, these dragons would likely hunt small to medium-sized animals, probably around 10 to 25 kg. They would likely be unwilling to tackle a human-sized animal as prey, and would likely flee from a single adult human. However, small children would likely present the temptation of an easy meal.