This here my friends is the Papapaponker. This type of critter was created when not all the Pinkie Pie's went in as into the mirror pool as cleanly as Twilight thought. A few that struggled hard enough managed to survive in various deformed states with one such Pinkie having enough of her original programming to continue to self replicate herself through the pool. Undetected by Twilight Sparkle after she simply sealed off the minor pool with a large boulder, they continued to slowly spread throughout the cave system and eventually into the wider world.
This is more of a horror prompt than fun posting isn't it?