Shrewtooth is this skinny ass bitch who's afraid of his own shadow. For revenge, he slaps people with his dick because he was slapped by Twolegs. It is not know if it was a dick or something else. He poops in the river because he's afraid that Twolegs with find his poop and clone him and become a robot like Firestar was. So the poop gets washed away so the Twolegs or anything else can't get to it.