Spark

Sparks are orb-shaped creatures who continually emit pulses and flashes of electricity. They are mostly found crawling around walls and ceilings and moving across wires. Sparks are the first in a series of enemies in the Mario franchise that are balls of electricity, including Li'l Sparkies and Amps.

Donkey Kong Jr.
Sparks first appear in Stage 3 of Donkey Kong Jr. Yellow Sparks (also known as Globes ) continue to circle their own platform, but blue ones can travel to other platforms through wires. These Sparks are generated by Mario in order to shock Donkey Kong Jr., preventing him from saving his father.

Donkey Kong
In Donkey Kong on the Game Boy, faceless Sparks always appear on ropes. They travel from left to right, and spawn at regular intervals. If Mario touches a spark, he will lose a life. They are indestructible.

Super Mario Bros. 2
Sparks also appear in Super Mario Bros. 2 as uncommon enemies, usually encountered underground or in buildings. They circle platforms, though some can hover in the air if a Mushroom Block is pulled out from beneath them. Sparks can be defeated by throwing an object at them or using a POW Block near them. Sparks can't be jumped on and can hurt the player's character by simply touching them.

Sparks changed slightly in Super Mario Advance. If the player is hit by one, they would be knocked back and the screen would flash. In addition, an item called the Spark Chaser can be used to expel them from new vase interiors.

Super Mario Bros. Super Show!
A Spark makes only one appearance on The Super Mario Bros. Super Show!, in the episode "On Her Majesty's Sewer Service". Having been dumped into the Tunnel of Doom, Mario and Luigi, while running from a Goomba, encounter a Spark that is quickly approaching. Mario and Luigi manage to avoid this Spark and the Goomba by creating a grappling hook out of a super spy hose nozzle and mini plunger; apparently unable to stop moving, the Spark and Goomba subsequently crash into one another, causing an explosion. In this appearance, the Spark featured does not appear to radiate any electricity.

Nintendo Comics System
Sparks are featured quite prominently in issues of Nintendo Comics System, appearing in such stories as "The Legend", "Bedtime for Drain-Head" and "A Mouser in the Houser". They are shown to be intelligent and fully capable of speech.

Mario vs. Donkey Kong
In Mario vs. Donkey Kong, blue, faceless Sparks periodically travel through wires just as they did in the Game Boy Donkey Kong.

Mario vs. Donkey Kong 2: March of the Minis
Sparks appear in Mario vs. Donkey Kong 2: March of the Minis. This time, they only travel along magnetized objects. They specifically appear on the Magnet Mania floor, where they slowly move clockwise from the red side of metal blocks with an "S" engraving. If a Magnetic Block is disabled from underneath it, the Spark will vanish and another one will soon generate from the block. They resemble their appearance from Super Mario Bros. 2.

Mario vs. Donkey Kong: Mini-Land Mayhem!
Sparks (now known as Magnet Sparkies ) reappear once more in Mario vs. Donkey Kong: Mini-Land Mayhem!. They act much as they do in their previous appearance, except they move at a slightly faster speed and briefly retreat into their blocks (now known as Magnet Sparky Generators ) upon touching it rather than passing them by. They appear in Cosmic Adventure, and are a solid blue rather than flash yellow.

Trivia

 * In Super Paper Mario, Tileoid enemies act just like Sparks.
 * Despite being an enemy in Yume Kōjō: Doki Doki Panic, Spark did not receive artwork in the manual until Super Mario Bros. 2. This is also the case for Tweeter and Flurry.