Drogo is hidden away at Masyaf. He’s the largest of all the Dragons, bigger than Draco.
(Dilios Note: I’ve not seen him to be able to tell you how big it is, but Rhys has stated Drogo’s brother was the one St. George defeated, and he remembered the Black Dragon as huge and filled the sky, with the ability to destroy armies in seconds with his fire)
He has the ability to camouflage into mountains, and mostly looks like a mountain or a crop of rocks.
Dastan has stated he has protected Masyaf, and he’s known to a few of the Assassins. They protect him as much as he tolerates them nearby.