The “Wings” of the Air Guard:
1)
Solar Wing, the traditional white coat/golden armor Pegasi
stallions that serve Celestia. They, like their Princess, watch over the day, and rise with the sun each morning.
2)
Lunar Wing, comprised of the rare, nocturnal bat-winged
stallions that serve Luna. Predictably, they watch over the night, and rise with the moon.
3)
Dusk Wing, an eclectic mix of ponies, with mare and stallion Pegasi of many different talents, several bat-winged Guards who used to work for Luna, and a few Griffons thrown in for good measure. Despite their varied origins, this small group of fliers is very tight-knit, and completely loyal to Twilight Sparkle. Also, because their Princess is not technically part of the day/night cycle,
Dusk Wing acts on a very case-by-case basis, and is the most likely to, say, chase down some thieves in a ponyhunt, or have its members go undercover in places like the Undercloud of Los Pegasus.
All three are lead by a Wing Commander, who is directly appointed by, and reports directly to, each respective Princess. The Air Guard’s Unicorn and Earth counterparts are known as the Magic Guard and Ground Guard, respectively, and are similarly separated by princess (the separate groups are known as
Solar Horn, Solar Hoof, Lunar Horn, Lunar Hoof, Dusk Horn and
Dusk Hoof). In general, the three Solar, Lunar and Dusk groups cooperate, but only within their spheres (e.g. Lunar Horn never watches the day, but it does work together with Lunar Hoof and Lunar Wing to patrol the night).
All together, these three branches (Air, Ground and Magic) comprise the full Equestria Royal Guard, which answers to the Princesses collectively, but there hasn't been an event that required all of the forces of the Guard at the same time since before Twilight's coronation (the last one was Discord's return).