In the comics, nobody knows Pigpen’s name.
My theory is that his last name is Thigpen, but people keep mishearing him because of a speech impediment.
You don’t hear it in the television specials because they didn’t do that kind of thing back in the Sixties and Seventies.
Maybe they’ll do it now and call it a diversity and inclusion effort?
While race-swapping half the characters, including Charlie Brown’s sister Sally.
Maybe Charlie Brown’s mom had a thing for Franklin’s dad or something.
Make Snoopy trans, self-identifying as a cat, and reboot Pigpen as gay.
(Which would explain the lisp.)