I think that a Pterasaur or a Megaras wouldn\'t be useful if you want to go through a forest or a jungle. The trees don\'t allow them to land, and even if they fly low they wouldn\'t see much because of the trees. A horse would be more useful here.
Also, a flying creature would not be able to get inside a cave or tunnel, if there are any. Although I don\'t think a horse would agree with going inside, unless it trusts the owner a lot. Otherwise, they would refuse to enter.
Horses are good for a quick tour around town too. If you are on a flying creature, you are very likely to miss certain spots if you don\'t know the town well.
Pterasaurs would have a very hard time in cold weather, due to their reptilian nature (they probably are cold blooded). Perhaps they tire easier when flying on those regions, or even die. There should definitely be downsides for riding a Megaras too, but I really cannot think of any right now.