Nyoooooo (number of o's vary) is a more recent gag from the Creatures, used in any situation where they want to childishly protest something, most usually said by Nova.
History
The joke was started by Nova in his MINECRAFT FINISHED 1.0.0 rage video, in which his mother attempted to take away his keyboard because of his "bitching" and yelling. Nova protested by whining "nyoooooo" and telling her she was taking Notch's side. The phrase has appeared a lot after this inlcluding Creature Minecraft videos and Creature Talks.