Originally, Peepo was going to be called 'Sparky' and Loki was named 'Peepo'. The names were changed early during the production.
According to Ric Carrott, his character, Chris Gentry and Pamelyn Ferdin's Laura were originally written to be twins but because of their obvious height difference this was soon changed to an older brother/younger sister relationship.
In the book "Creating the Filmation Generation", Lou Scheimer revealed that Peepo was one of the few props he kept from any of his productions and that it found a home in one of the Scheimer bedrooms.
Peepo was operated by John Berwick on set and he also provided the temporary dialogue for the robot and any other characters that would be looped over later by Erika Scheimer and/or Lou Scheimer.
Although E.C.S. sounds like a expensive voice synthesis program to create Peepo's voice, this actually stands for the initials for the voice actor for Peepo: Erika Caroll Scheimer.