Fighter Fly

A Fighter Fly (also spelled Fighterfly ) is a grinning, moth-like pest that first appeared in the sewers of the original Mario Bros. arcade game. They are characterized by their ability to hop – an ability that allows them to cause trouble for Mario and Luigi.

Mario Bros.
In Mario Bros., Fighter Flies are one of the myriad of pests that emerge from pipes to attack Mario and Luigi. They first appear in Stage Six. After a Fighter Fly emerges, it hops across the stage's platforms. As such, Mario has to precisely time his Jump when the Fighter Fly is on the ground to flip the creature over. If airborne, a Fighter Fly cannot be flipped over - not even if Mario uses the POW Block. Once flipped over, Mario has to run into the insect to clear it off the stage.

In other iterations of Mario Bros.
Fighter Flies appear in Luigi Bros., an additional game featured in Super Mario 3D World. Like the other enemies in Luigi Bros., Fighter Flies behave in the same way as in Mario Bros., which this game is based on.

Fighter Flies are present in NES Remix as enemies and level objectives in some of the challenges and remixes based on Mario Bros..

Super Mario Land
In Super Mario Land, Fighter Flies are simply named Flies and are seen in the Birabuto Kingdom of Sarasaland. In this game, they hop towards Mario, just like in Mario Bros.. Kumos from the Easton Kingdom attack in the same manner as Fighter Flies. They can simply be jumped on or be shot with a superball to be defeated. Flies award Mario with 400 points when he defeats them. They are only found in the first stage, World 1-1.

Mario & Luigi: Superstar Saga
In Mario & Luigi: Superstar Saga, they make a reappearance. They live in Stardust Fields, the border between the Mushroom Kingdom and the Beanbean Kingdom. To attack, a Fighter Fly hops towards Mario or Luigi and tries to bump into the plumber. The heroes can jump to avoid the attack.

Mario and Luigi also encounter a stronger version of the Fighter Fly, the Super Fly, in the basement of Beanbean Castle.

Super Smash Bros. for Wii U
Fighter Flies make a cameo appearance as a trophy in Super Smash Bros. for Wii U. This trophy is shown below, along with the in-game description.

Super Mario Maker
A Mystery Mushroom costume, designed after Fighter Fly, can be unlocked in Super Mario Maker (after the version 1.20 update) by completing the hard version of Gnat Attack. When performing a jump as Fighter Fly, the player can hear the high-pitched noise this enemy made in the original Mario Bros.. If the player presses while playing as Fighter Fly, it will turn into a Freezie, which is a foe that also debuted in Mario Bros..

Names in other languages
Mario Bros.

Super Mario Land

Super Mario All-Stars

Mario & Luigi: Superstar Saga

Trivia

 * In later remakes of Mario Bros., the Fighter Fly was redesigned. It is still thin, but it has a different mouth instead of the grinning face. Its eyes also now resemble a Lakitu's goggles.
 * Fighter Flies also appear on Mario's Classic Court in Mario Power Tennis. Like other enemies, they can trip up an opponent.
 * For reasons unknown, the Fighter Fly is absent from the stage, Mario Bros., in Super Smash Bros. Brawl.
 * The name of the enemy is a pun on the insect firefly, and the word "fighter", which is another name for a boxer.