Parasprites

Parasprites are insect-like creatures that first appear in Swarm of the Century. The name "parasprite" is a portmanteau of parasite and sprite, a mythical forest creature from folklore. Lauren Faust had compared them to tribbles from Star Trek, whose voracious appetites and rapid multiplication turn them from pets to pests.

Depiction in the series
A parasprite infestation in Ponyville is the subject of the episode Swarm of the Century. Fluttershy encounters a single parasprite, who chirps at her and endears itself to her. When she tries to feed it, it eats an entire basket of apples in mere seconds. Fluttershy unwittingly brings it into town to show to her friends, who are all unfamiliar with the creature and become enamoured with it, except Pinkie Pie who storms off in search of musical instruments. The ponies' adopted parasprites become huge swarms overnight; it is only then that the ponies discover that parasprites can quickly reproduce by coughing up another parasprite. The pests relentlessly devour crops and food, but are able to consume wood, fabric, and inorganic materials after Twilight Sparkle casts a spell to stop them from eating food. The ponies make several attempt to round up the parasprite, but the pests keep getting loose. Twilight runs to Zecora's hut and consults her, but Zecora offers no solution.

"Tales of crops and harvests consumed. If these creatures are in Ponyville... you're doomed."

- Zecora

Right before Celestia's arrival at Ponyville, Pinkie Pie appears with several musical instruments that she had been collecting throughout the episode from her first encouter with the parasprites. She furiously plays on the instruments, and much like the Pied Piper she leads the parasprites into the Everfree Forest in a line, passing by her friends and Princess Celestia, who are amazed at the sight. Princess Celestia thanks Twilight for the "parade" in honor of her visit, but says she has to attend to an "infestation" of some sort in Fillydelphia and cancels her scheduled visit to Ponyville, much to Twilight's relief.

Other depictions
Parasprites are shown in the Discover the Difference game. Five of them appear in the Swarm of the Century image.

Five parasprites, along with a host of characters from the show, appear on the Comic Con 2011 promotional poster. There are two on its left side and three on its right.

Parasprites in groups of seventeen are one of the four types of obstacles in Rainbow Dash's Rainboom Game. The in-game How to Play instructions show them in a group of three, and the Hubworld website's How to Play instructions refer to them as balloons.

A swarm of parasprites is shown in the Ruckus Reader interactive storybook Things That Go Bump in the Night. They appear in the imagination of Applejack, who refers to them as "mean bugs [who] want to eat our food".