Noooooo (the number of o's vary) (pronounced with a Y sounding syllable [Nyooooooo]) is a more running gag from the Creatures, used in any situation where they want to childishly protest something. It is most usually said by Nova.
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, including in Creature Minecraft videos and Creature Talks. The phrase has not been used in more modern content, but will always be remembered as being an iconic quote.