In 'The Hedge Knight' ancient dragons are mentioned, thousands of years olds. Were there Dragons in Westeros before the Targaryens brought them, or did the Targaryens bring the skeletons of the old Dragons with them?
There were dragons all over, once.
The follow up question, which I realise may be something you keep for the books, is what happened to the Dragons out of Westeros? If I understood correctly, the Alchemists say that there were no more Dragons anywhere. Was that so?
There are no more dragons known to exist... but this is a medieval period, and large parts of the world are still terra incognita, so there are always tales of dragon sightings in far off mysterious places. The maesters tend to discount those.