The Easter Platypus, in the Invader Zim universe, serves as a folkloric figure and symbol of Easter that brings "Easter Shrimp", unlike the more traditional Easter Bunny that brings Easter eggs.
Similarly to Santa Claus, in some versions the Easter Bunny was also meant to bring gifts to children on the night before their respective holidays. However, it can be assumed that the Easter Platypus only gives shrimp out for unknown reasons.
Whether Zim created the character or it was already a part of Earth's culture is unknown, as it has been shown holidays in the Invader Zim universe are vastly different than in real life, like Valentine's Day, which due to an unknown event (one that Ms. Bitters stated to have been unpleasant) has replaced cards and candy with heart-shaped slabs of meat. So, it's possible that a similar event in the past caused the Easter Bunny to be replaced with the Easter Platypus, as the people in the North Pole (and more surprisingly, Dib) didn't question its existence.
- Assuming that the Easter Platypus is an actual holiday figure, it's unknown for how long he has been around or what caused him to replace the Easter Bunny.
- Zim dressed as the Easter Platypus towards the end of "The Most Horrible X-Mas Ever" as a way to get the population of Earth to beat up Dib for launching Santa into a space, most likely as an opportunity for revenge against Dib even though he technically saved him from being trapped in the Santa Suit forever.