Milt/Sheldon is the Pygmy Hippo Bean Scout in the background in Camp Kidney.
In Campers All Pull Pants it is revealed his name was Milt. However, in Burpless Bean, Lazlo and Slinkman refer him as Sheldon. It is unknown if Sheldon is his real name or nick name. In Being Edward, Lazlo refers him as "Hippo guy". In Edward's Big Bag, he is referred as Milton. Probably a full name of "Milt".