Popopo was the original main character of the Kirby series, but was updated into Kirby.
Popopo was originally going to be the protagonist of the Kirby series (that would have been called the Twinkle Popo series), but due to issues with making his in-game sprites work, Sakurai then favored a placeholder ( Who would then become into the famous Kirby ) that he had created over the original design, and so Popopo was eventually scrapped along with other original ideas for Dream Land.
His name was meant to be a parallel of Dedede.
Popopo has never officially appeared in-game but he makes cameos in the Kirby's Dream Collection's Celebration Book, where the booklet describes the original ideas for Kirby's Dream Land where Popopo was the main character in the few images. He also had additional appearances in early ads for Kirby's Dream Land. .
Kirby, Popopo's successor
Early images of Popopo and early Dream Land from the Celebration Book.