In my book series, there is a planet called Aurea that is home to all sorts of strange fauna. Not only does it have animals from the real world, but it also has prehistoric and mythical creatures as well. One of them is the Argentavis, a Miocene-epoch vulture with a 30-foot wingspan and weighing up to 180 lbs. The Aureans have domesticated them, originally for their use as essentially giant hunting birds. However, they have been selectively bred so they are slightly larger, now with a 40-foot wingspan and weighing 200 lbs.
Would these creatures be able to be ridden by humans for flight purposes (and if not, how much more would they have to weigh for this to be possible?), and would keeping these creatures tamed (by a medieval-age society by the way) even be feasible?