I’ve looked through the Monster Manual for clues but I can’t seem to find anything concrete about how much a pteranodon (or any other flying creature for that matter) can lift while flying. They have a fifteen- to twenty-foot wing span, and are medium beasts (MM, pp. 79–80) so it seems reasonable to me that they could carry a medium-sized PC. But that seems like it might be a bit overpowered. Is there any rule for how much a flying creature can lift in 5E?