During the ending cutscene, you can see D'Artagan peeking out of the outhouse at one point. This was originally going to be the main character in Psychonauts, but ended up evolving into Raz. Information from developers and such would imply that "Dart" was a fully animated and working character before the decision was made to rework him, which is most probably why he was implemented in the ending cutscene as a nod. You can see concept art for him
here,
here,
here and
here. This is a low-quality screengrab from the ending cut-scene:
I don't remember who got this screengrab, but thanks regardless. ;