Date: 03/30/2013 2:06 PM Title: Demon From the Moon
Jinpei is wearing fundoshi, a sort of loincloth. Before Americans introduced Western-style undergarments, Japanese men wore them.
I am surprised that the Sandy Frank people didn't find a way to color in Keyop's rump. Perhaps they figured nobody would notice VCRs were expensive and thus uncommon at the time BotP aired (and probably not wasted on a cartoon), so few could check even if they did spot G4's naked bum.