What if dragons were like any other species of animal in todays age and had many different variations spread across the world? Ranging from the size of small chihuahuas to the size of a large jumbo jet. Some being non violent or even friendly to those that if aggravated would become hostile. How would they really be treated?
This would vary a lot depending on the prior relations between humans and dragons, and if those dragons are sentient and if they possess intelligence or not. And the size of the dragons!
In any case, we should keep in mind that the dragons wouldn't pop up out of nowhere. If they exist, they probably cohabited Earth with us for a long time.
Friendly, small and non-sentient dragons would be great pets, don't you think? I would definitely keep a dragon as a pet.
Aggressive and non-sentient dragons would be treated as any other animal, kept in the wild and probably hunted down if they become a threat. Probably some of those in the zoos.
Friendly-neutral and sentient/intelligent dragons would get their own societies, and humans would've been in contact with them since ancient times, unless dragons kept themselves hidden for some reason. In that case, probably commercial trades, alliances, cooperative efforts, wars...
Aggressive and sentient dragons would probably wipe out the humanity. Particularly if they're the size of a large jumbo jet. And in the exceptional case that humans somehow survive, there would be war or humans would be hidden, trying to avoid dragons, because definitely we couldn’t have developed a modern-nowadays society if dragons were opposed to us.
It depends a bit on what the dragons eat and whether they are threatening and/or valuable to humans' interests.
Assuming that you're talking about a 'traditional' dragon which is reptilian, fire-breathing and carnivorous, then it would at the very least be considered a pest predator by pastoral farmers who would not like dragons stealing their cows/sheep/goats/fish/etc. Wild populations of medium-sized dragons would be hunted away from populated areas for this reason alone.
Large dragons are very likely to be considered trophy animals, and given the fire symbology will probably also develop aphrodisiac lore in traditional medicine, so large species are likely to be hunted unsustainably for these reasons.
What it probably doesn't depend much on is the actual temperament of the dragons, especially whether they are actually threatening to humans. Humans are good at talking up the magnitude of external threats...