Otto the Grouch

 Otto the Grouch   is  Oscar the Grouch 's baby brother. He's also the baby brother of Ernest the Grouch  &  Oscar 's sister  Bunny, making him the 3rd son of  Mr.  &  Mrs. Grouch. Otto is both seen & mentioned in the 1976 book How to Be a Grouch .

Books

 *  How to Be a Grouch  (1976)