Earlier versions of the scene in which Po eats the disgusting soup also included the kids trying it, and describing the taste "like a great-grandma's feet," "like dried scabs and cilantro!" and "melted sadness poured over a pile of hate."
Fun Fact: The actor who plays Rooster - Michael Rivkin - actually went to school with the actor Jack Black, who originated the role of Po - and they remain friends to this day. (Michael's comedy rock-band "SuperFloss" occasionally tours with Jack Black's comedy rock band, "Tenacious D.")
Rooster was introduced in this episode to "out" Po and challenge him to fight in a moment where he and the kids are trying to fly under the radar. The character and his Warthog henchmen (Wing and Wong) became so popular with the writers - especially after the casting of Michael Rivkin - that they decided to bring him back. Again and again and again.
Alternate "gang" names that the writers came up with in the writer's room for "The Rooster Brothers": "Heaven's Devils," "Heck's Angels," "The Sunshine Boys," "The Raspberry Berets," and "Soldiers of Fortune Cookies."
The innkeepers (at the inn populated by thieves and ne'er-do-wells) are inspired by Basil and Sybil Fawlty, the lead characters from the hugely popular British comedy, "Fawlty Towers" starring John Cleese.