This man here is Not Stu. He looks a lot like Stu Pickles from a rear end view. He originally appeared in the Rugrats episode 'In the Dreamtime'.
One night, Chaz Finster said "Good night" to his son, went out of his room and saw Stu (but he's Not Stu!). He is Da-Da's arch enemy, plus he is also known for raping people in their dreams.