Corrupt Penguin

Two things make one. Let a Blood Moon rise in a Corruption world and the penguins already wandering the snow coast flip hostile on their own, no input from you. Or skip the wait entirely: hit an ordinary Penguin with Vile Powder and it converts on the spot. The powder is the route to use if you want the bestiary slot filled tonight, since it ignores the moon and the calendar. A Crimson world gives you the Vicious Penguin under the same rules instead.

They arrive with the Blood Moon and then refuse to play along with it, since Bloody Tears do not drop from these at all. A coast packed with penguins is a poor place to spend the event if that was the plan. The one real draw is the Pedguin vanity set, which the untouched penguin cannot roll.

Every other critter the Corruption gets hold of comes back purple. This one comes back bloody, which is the look you would expect from the other side of the world evil.