Cosplay

Cosplay, or "Costume play" is the act of individuals usually dressing as fictional characters, typically from Television or Comic books, though cosplaying of historical figures is not unheard of.

Friendship is Magic
Cosplay was invented during the Age of Heroes by Starswirl the Bearded who loved to dress up as his hero, Wilhelm the Wacky Wizard and he only smelled just a little.

Although typically seen as an aspect of "Nerd culture", cosplaying is quite popular throughout Equestria. On Nightmare Night, many ponies practice a form of cosplaying by dressing up as various archetypes or fictional characters. Sometimes however, they dress as historical figures instead, such as Rainbow Dash dressing up as a Shadowbolt and Twilight as Starswirl the Bearded.

At Comic Con, Spike's cosplay as the Starfleet Captain won him the Cosplay contest.

During their stint as detectives, the Cutie Mark Crusaders traveled to Comic Con. While there, Scootaloo investigated cosplayers for Deadpool, Death of the Endless and Harley Quinn.

Equestria Girls
During her brief stay in the Anime club, cosplayed as an Unknown cape wearing anime character. cosplayed as as she role-played with her friends in the Role-playing club. Likewise,, and  also cosplayed as their characters.

Behind the scenes
Cosplaying is a very real thing indeed and has direct origins in the 19th century of real-world chronology, but similar concepts have existed far beforehand.

Trivia

 * Although Twilight Sparkle once cosplayed as Starswirl the Bearded, Starswirl himself actually based his look off of Wilhelm the Wacky Wizard.
 * In the Equestria Girls continuity, Trixie seems to cosplay as Starswirl instead of Twilight.