Did dragons exist?
Coming from a religious view point, a resounding yes. They are gods of the old religions, some are considered "demons" (I use the quotes because I don't believe in a being of pure 'evil', nor pure 'good'). Can it be proven or disproven as to physical manifestation on earth? Not overly I stress the disproven part of the question... just because we haven't found hardcore evidence either way yet doesn't mean we won't in the future. You cannot say they don't exist and still say you believe in any of the other thousands of gods because they fall under the same situation. And no you can't pigeon hole gods into "hey this one is similar to this one so they are the same being" because contrary to popular belief, not all religions believe in the same ethics/morals/stories.
But, this isn't a religion topic.
You can't mention someone's gods and say it's not a religious topic... This goes for ancient humans, those who follow ancient religions, and especially those who follow one of the dragon-centric paths.
Dragons are fire-breathing lizards
OK fine maybe the Western dragons are depicted as such, but there are more than one country and view of dragons out there. They are various forms, found in various climates and environments, and have various traits. The only thing that is a common element of all dragons world-wide is that they are composite creatures.
Asian dragons are depicted as having the head of a camel, horns of a stag, eyes of a demon, neck of a snake, belly of a clam, scales of a carp, claws of an eagle, soles of a tiger, and ears of a cow.
Western dragons are large lizards with bat-like wings.
Amphiteres are dragons who are essentially bird winged serpents.
Hydras are multi-headed dragons.
Sumerian dragons much resemble fu dogs mixed with gryphons.
Caribbean dragons are large serpents.
Nagas are often depicted as dragons, which are humans with snake bottoms who can also change into full snake.
The dragons you speak of are, as stated before, a symbol to evil....
Dragons being solely symbols of evil is a relatively new concept. Most of the dragons, even the stories of hoarders don't show evil.
The "treasure" they hoard is not normally declared. It can be shiny, could also be a clutch of eggs, another dragon, perhaps a person, or even wisdom. Even the act of protecting their hoard is a sign of guardianship over evil. As for them attacking intruders, any being (animal, god, human, otherwise) would attack intruders to protect what's their's, esp if it's progeny.
Also chaos and destructive acts do not mean evil. Pure peace in the sense of how it's preached now-a-days is stagnation. Stagnated energy needs to be freed so that things can be created. Therefore destruction is just as needed as anything else.
Examples of some good dragons are:
The asian celestial dragons chase the pearl of wisdom through the air. Asians also have stories of serpents and koi who can transform into celestial dragons after a life of virtue, wisdom and good deeds.
Abrasax was a hydra who was a composite of a two legged dragon and serpent. He is the lord of 365 virtues.
Agunua was an amphitere creation god.
Aido Hwedo is a serpent who is a creation god as well as supporter of the earth.
Tiamat is a sumerian creation goddess. Yes she raised against her children, but only after they killed her husband. She suffered their imputence much longer than her husband did out of motherly duty.
Anata is a creation/destruction hydra. (creation/destruction as in the cycle of life)
Muchalinda was a cobra, king of the nagas, who wrapped around Buddha protecting him while he was meditating under the Bo tree.
In the Philippines there is lore of a dragon who was destructive to all but the one girl who did not try to hurt her. The one who showed the dragon clemency gained it's protection. It would raise the tide against those wh

uld hurt her, etc.
mean, god did put all animals on the earth to fulfill a purpose, whether to be food, help humans, or comfort us (dogs, cats etc...) so why couldn't they're be another type of animal in charge of this kind of thing
How could god put dragons here when the creation gods were dragons? Well goddess, since Abzu was never declared what his form was, but many draconic sects view him as a dragon.... but Tiamat bore the first of the gods, who subsequently created humans later on. They pre-dated Judaic religions and their version of god...
...they would not be domesticated and would have killed whatever got in its way.
Thel saying "guardian" is not saying "hey look at me, I domesticated a dragon!" Like all spiritual guardians, dragons can choose to teach a human if they so choose. Also like other gods, guides, teachers they can and will make your life a living hell if you don't follow what they suggest.