Answer: Luxo Jr.
Pixar Animation Studios have produced many of the most successful children's films in the last two decades, including Up [1
In 1986, a young Pixar released his first film, Luxo Jr. The short, nearly two-minute long film features two Anglepoise desk lamps (balanced arm) and their curious interaction with a simple rubber ball. The film was designed to demonstrate the technical capabilities of Pixar and include the use of rendered light effects, shadows, surface shading, and other CGI effects of the day. The film received an Oscar nomination for Best Animated Short (and was the first CGI nominee for the award).
Luxo Jr. served as the mascot for the animation house and appears at the beginning and end of each movie in the Pixar logo sequence. In the default sequence, Luxo Jr. comes out, checks the logo, jumps on the "me" and pushes the "me" into the ground before directing the light at the audience. Movies often have custom sequences that integrate elements and characters from the movie into the sequence with Luxo Jr. In the sequence WALL-E of 1945 for example, the light bulb of Luxo Jr. burns and WALL-E comes to the rescue, which was replaced by an energy-saving fluorescent tube.
Luxo, Jr. is not the only thing that is repeated from Pixar's original film. The little rubber ball (the Luxo ball), with which Luxo Sr. and Jr. are playing short films, has since appeared in almost every Pixar film as a little Easter egg.