Time for even more headcanon-worldbuilding fun! 8D

(May contain light PC spoilers.)

Unicorns are traditionally sorted into different classes of mages. Usually, their cutie mark determined what type of mage you are.

Art Mages had a talent in the arts, especially in sculpture and theater. These unicorns were meticulous, but also masters of improvisation. Historically, they were responsible for building and designing Equestria’s grand cities and castles. (Ex. Sweetie Belle is an art mage of the music subclass.)

Earth Mages had talents with the use of the earth’s minerals. Legends said that gems had special powers that ranged from bring two ponies to love to curing ailments. Early earth mages were doctors and alchemists. Modern ones are mostly gem-cutters, merchants, and designers. (Rarity is an earth mage of the design subclass.)

Cosmic Mages were observers of the stars and the universe, as well as experimenters in the field of magic. Most modern spells were written and tested by these unicorns. Today, true cosmic mages are hard to come by. (Twilight is a cosmic mage.)

Illusion Mages were entertainers and mentalists. Unlike cosmic mages, they tended to experiment only to amaze and bewilder their audience. But on the flip side, they pioneered in the study of behavior and mental states. (Trixie can be considered an illusionist.)
(Thanks to askmedusapony-mod for this edit.)

Light Mages were responsible for bringing light to proto-Equestria and keeping the sun lit. They were also experimenters and assisted in the creation of modern clocks and calendars. Today, they are considered a defunct class. (In PC, Dinky is a light mage.)

Dark Mages are probably the most mysterious of the classes. They are attributed to nightfall and shadows, but not much else is written about them. Old legends hint that they simply faded away into obscurity and are responsible for Equestria’s turmoil.