Last Updated: 8/22/2017
This easter egg can be seen in all versions of the movie.
Early on in the movie when Remy is down in the sewers and is told by the illustration of Gusteau to go above ground and look around, Remy runs through a number of peoples houses in order to get to the rooftops.
Shortly after he encounters the couple who fires a gun into the ceiling, he runs up a pipe and into a house where he’s startled by a dog, though we can only see it’s shadow. Upon closer inspection, this particular shadow happens to belong to Dug, the talking dog from UP who accompanies Carl and Russell through the jungle.
The cool thing about this easter egg is that Ratatouille released almost 2 years before UP, making it a prediction easter egg. Pixar is known to put these types of eggs in almost all of their movies, as found here.