A typical parasprite.
Parasprites are small, insect-like creatures that appear to be native to the Everfree Forest. They look very much like little furballs with wings, and practically every pony who comes across them finds them cute. Despite their adorable looks, parasprites are huge pests, able to eat huge amounts of food non-stop. They also multiply at an exponential rate, reproducing by coughing up another parapsrite once they've had enough to eat.
The parasprites were first brought to Ponyville by Fluttershy after she discovered one near the edge of the Everfree Forest in the episode Swarm of the Century. Though she only found one parasprite, by the time she made it to Sugarcube Corner where Pinkie Pie and Twilight Sparkle were preparing for Princess Celestia's impending visit the next day, there were now three of the creatures. Twilight Sparkle was as entranced by the parasprites' cuteness like Fluttershy was and offered to take one with her. Pinkie Pie declined, however, recognizing the creatures as parasprites, and left to look for a trombone.
The single parasprite with Twilight Sparkle had multiplied to three by the time she reached Carousel Boutique, which confused her. Rarity and Rainbow Dash, who were in the boutique at time time, also found the creatures adorable and offered to take one each. It proved to be a bad decision, however, as overnight the parasprites had multiplied into dozens, if not hundreds, of the creatures. Discovering their problem, the ponies tried herding the parasprites back into Everfree Forest with the help of Applejack's herding skills. Although they were succesfull in leading the parasprites away, Fluttershy had secretly kept one, which just started the whole cycle all over again. In the meantime, Pinkie Pie was searching around and collecting musical instruments.
The plague of parasprites descended on Ponyville with their voracious appetite, eating every food in sight. Desperate to stop them, Twilight Sparkle cast a spell to stop the parasprites from eating all the food. The spell backfired horribly as the parasprites now started ignoring the food and began eating the town itself, leading to widespread panic as every pony left to defend their properties. Twilight Sparkle even tried asking Zecora for help, but although the zebra recognized a parasprite for what it was, she did not know how to stop them. Seeing the destruction of Ponyville and knowing that Princess Celestia was due to arrive soon, Twilight Sparkle had a nervous breakdown.
Just before Princess Celestia's arrival, Pinkie Pie appeared, playing like a one-pony band with all the instruments she had collected throughout the episode. The parasprites seemed hypnotized by the music and started following Pinkie Pie as she marched along towards the Everfree Forest, the rest of her friends following along curiosly. They met Princess Celestia at the edge of the forest outside of town as she arrived, where she mistook Pinkie Pie and the parasprites as a parade in honor of her visit. Fortunately for Ponyville, Princess Celestia had to cancel her visit due to an unknown infestation in Fillydelphia.