Plague Minions are jumpy minor enemies that throw explosive vials at Shovel Knight that damage him and destroy blocks when they explode. They can be found in the Explodatorium and the Tower of Fate.
Their roles are greatly expanded in Plague of Shadows, where they serve Plague Knight, aiding him in the quest for the Ultimate Potion. They often give gameplay hints, though they are quite goofy and somewhat incompetent. Comically, Plague Knight can blow them up in the Potionarium and other places where they can be spoken to.