The Onion Replica is one of the 201 treasures in Pikmin 2. It is found in the Perplexing Pool. To get it, split up forces between your captains. Have one take a small group of white Pikmin to the dead end behind the ship while the other takes a large group of blues out of the landing site and left around the concrete wall into the water. Kill the Water Dumples and then break down the black gate. Trudge through the water and kill the Yellow Wollywog and the pair of water dumples. Ignore the Skitter Leaf and switch to the control of the captain with the white Pikmin. Have him toss the whites onto the small indent for the other captain to call. Lead them onto the tip of the peninsula and they will unearth the Onion Replica, which is ironically named after the Pikmin's onions, which were named after this real onion. Toss the whites back up to safety and let your blues carry it through the water and back to the ship.
"This looks just like one of Hocotate's famous sweet onions! Come to think of it, I gave the Onion its name because it looked just like a Hocotate onion. To avoid confusion, I'll call this vegetable an Onion Replica."
"This plant closely resembles the pride and joy of Hocotate. The comfort of its familiar form and the surprise of its unexpected taste mingle to form... veggie harmony? Try it in some hot soup today!"