As Remy runs through an apartment he is startled by a barking dog. We only see the dog’s silhouette, but it’s unmistakeably Dug the Dog from Up:
Believe its already been found and commented on… here in
The Definitive Pixar In-Jokes List
But year noticed that in a viewing of the movie a good few months back.
Hard not to when Brad went out of his way to point him out in the commentary…
Never watched the commentary actually… 
But I shoulda known you guys would have already spotted it…
Yeah, kinda old news. I noticed it during my marathon, it’s actually a lot more noticeable than I thought when I heard about it. Pretty cool, Dug’s awesome.
I liked Brad explaining [spoil]he’s in the movie after the next movie after Ratatouille. And also just remarking that the dog “makes an appearance” in the upcoming movie (instead of saying he’s a major character).[/spoil]