If there is one thing pegasi are famous for, it's their incredible xenophobia. Inherently extremely distrustful of the other races, pegasi are very isolationist and tend to prefer the company of even stranger pegasi compared to their non-pegasus family members. Most pegasi are part of the Flock, the unspoken cultural ties that every pegasus shares with each other. Within their own race, pegasi are astonishingly comforting, helpful, and selfless, going out of their way to assist other pegasi when needed and tend to their needs. Communities of pegasi are some of the safest, most generous, and well-kept communities in most Universes... at least, to other pegasi.
The core ideal behind the Flock is that pegasi, at their most base level, are affiliated with flight. Flight is the cornerstone of pegasus culture, and a pegasus that cannot fly (i.e. did not pass their flight test) is culturally shunned. It's not that everyone has to be the best, fastest flier - no, pegasi just have to have some flight capabilities at all. This allows wriggle room for those born with disabilities (small wing mutations, for example) to still fit in with the Flock, but shuns those who don't care about their race's characteristics and who would weaken the bonds within the Flock.
This adoration of flight is instilled in pegasi from a young age; between the ages of 16-20, teenage pegasi are expected to take and pass an extensive exam consisting of over 100 multiple choice questions and then a basic flight test afterwards, to be judged by three government-certified adults. Two of the five available grades, P and E, are considered "passing" and have no negative repercussions for either. However, G, A, and S are "failing" grades, and the pegasus is punished accordingly by having their cheek branded with their grade and wings clipped. In some societies, the passing grades have been extended to allow for G-grade students to avoid the punishment by retaking the test a year later, although this is not quite universal.
The test is often the focus of a young pegasus' life, spending their younger years prepping for it and pouring over the countless study guides and classes offered on the topic. The test itself is not extremely difficult; it involves a simple takeoff, loop, some mild weather manipulation, and then a landing. Still, the test is so over-stressed in pegasus culture that many of the failures are due to nerves and stress rather than an inability to fly. Pegasi who fail the test can still graduate high school or college, but will be ostracized from overall pegasus culture due to their cheek branding.
Another important pegasus culture facet is that of weather manipulation. Like all flying races, pegasi are able to sit on, build on, and live on clouds and in the sky, but only pegasi have mastered the art of creating weather to sell in factories and to control natural phenomena. Anyone can copy their recipes after they're first discovered, but pegasi are capable of manipulating weather with their bare hands and movement, making them especially equipped to deal with disastrous weather crises. Some people have reported that pegasi are capable of influencing phenomena that are not strictly weather-orientated, like tsunamis, and it's unclear where the line is. Most pegasi tend to go into weather manipulation because of this, but can also be found in any activity that requires their special set of skills.
Of the other races, pegasi find unicorns to be the most annoying and aggravating, and go out of their way to avoid them.
(artwork by @meatskeleton )