The duck at the end of "Nerdł Plays... Gone Home". It appears in similar fashion to Batman the teddie bear.
When playing Cookie Clicker, it starts out innocently enough, with Dan talking about the game and playing it. It then fades to a time-card and then fades back in, showing Dan addicted to the game with ominous music in the background. As the video fades to black, an agonized Dan can be heard.