In various media forms, when a goatee is on one of two twins, that is the evil one. Perhaps it started in Spock and Evil Spock in the Star Trek episode “Mirror, Mirror”.
Perhaps it comes from the Goat of Mendes:
Bender’s evil twin Flexo:
Cartman’s evil twin:
In Codename: Kids Next Door, Numbuh 4 has an alternate “Negative Numbuh 4” with a fake goatee because it “makes him look more eviler” (exactly what kids need, bad grammar in their cartoons):
Edit: Flexo is not bender’s evil twin, he is his good twin, as is with Cartman. Goateed Cartman is the good Cartman, and the rest of the South Park crew’s goateed twins are evil.
Tags: codename: kids next door, evil twin, futurama, satan, south park, spock, star trek