Nyan Cat is the name of a YouTube video uploaded in April 2011, which became an internet meme. The video merged a Japanese pop song with an animated cartoon cat with a Pop-Tart for a torso, flying through space, and leaving a rainbow trail behind it. The video ranked at number 5 on the list of most viewed YouTube videos in 2011.
In May 2013, Christopher Torres and Charles Schmidt, the creators of Nyan Cat and Keyboard Cat respectively, jointly sued 5th Cell and Warner Bros. for copyright infringement and trademark infringement over the appearance of these characters without permission in the Scribblenauts series of video games. Torres and Schmidt have registered copyrights on their characters and have pending trademark applications on the names. Torres released a statement saying that he had tried to obtain compensation from 5th Cell and Warner Bros. for commercial use of the character, but was "disrespected and snubbed" multiple times. The suit was settled in September 2013, with Torres and Schmidt being paid for the use of the characters.
Source: Wikipedia, "Nyan Cat", available under the CC-BY-SA License.