This Article Contains information directly from the Pikmin Games. This article is based off of an Enemy/Treasure/Area/Cave/Pikmin/Character from the Pikmin series of Games.
The Onion Replica is simply a real vegetable, an onion (not to be confused with the Pikmin Onions). It is found in the Perplexing Pool behind the landing site, and involves a complicated process to get it, one of the most complicated on-land treasures to get in the game. You must destroy a nearby black gate to get behind the landing site, then throw White Pikmin over the edge on the secret path behind the ship. Use the other leader to guide the whites onto a small peninsula, and dismiss them on the tip. They will detect a completely buried item and start digging it up, then you will need to change the Pikmin carrying it to Blue Pikmin so they can carry it across the water. Just throw your whites the same way you got them onto the peninsula, except onto the landing site area. Make sure you defeat the Water Dumples and Yellow Wollywogs along the way. This may seem complex, but you may easily figure it out on your own, it just involves simple problem solving. The Ship may have called it an "Onion Replica" referring to the fact that the Pikmins' vessels are also referenced by Olimar as Onions.