I don't know exactly how long it has been in development, but the first video of an unfinished Chill Penguin stage came out about a year ago, and this is all the progress shown to the public by developer TiagoSC since then. Chill Penguin is now playable from start to finish including the boss fight, as well as the intro stage complete with Vile Fight, and all of Armored Armadillo without the boss so far.