And once more, as a balance, we are left with the actual tyranny of the player, who wants to play whatever he wants, despite the DM maybe having prepared a very consistent world with some restrictions because they fit his cosmology, history, myth, cultures or whatever. Some worlds are created with the principle of "whatever exists in D&D exists in that world" and that's fine too. But the players are playing in the DM's world, so they have to accept his rules and his design. And if, for example, he does not want flying creatures because they are unbalanced compared to what he has prepared (and please don't server me the BS about "it's easy to adjust", first it's not true especially at low level and second, a DM has enough work as it is with all his players, why should he expend specific effort to cater just for my whims ?), then sorry, that's the way it is. We are just starting a new campaign in an interesting world, the types of tieflings there are different, so I'm choosing one of them. And I won't be an entitled little wangrod and insist that my DM allows me to play a winged tiefling just because they have to exist somewhere in the universe. Simply put, in that universe, they don't, and I accept that to be able to play in a consistent campaigns where tieflings are descendant of specific creatures, with a history and consistency, and I will enjoy that.
All of this goes with a trend that started with 3e of not respecting the work the DM does for his players enjoyment. Don't get me wrong, I'm all for the players having fun, but there are limits to it, respect needs to flow both ways, and respect for work and preparation is very important for me.