Foghorn, one of my fav characters, has been misattributed to a radio show character known as Senator Claghorn. He's actually an amalgam of the Senator and an earlier character called The Sheriff.
However, the first tracks of the first Foghorn's debut were laid down 10 months before the Senator character made his first appearance.
The Sheriff came from a radio show called Blue Monday Jamboree. That's where the I Say, the shouting over everyone, and the repeating what the character had said came from. But that specific character was hard of hearing and a lot of comedy came from that (I have read, never heard a show).
But the "that's a joke, son!" "he's too short. Short, that is" along with that bombastic voice do come directly from the Claghorn character.
Which, according to the actor, isn't a parody of southerners, it's an impression/direct copy of a Texas rancher who gave the kid a ride while he hitched across Texas. Apparently, he would start a sentence with "I own 500 head a cattle, boy, I say, I own 500 hundred head of cattle, and ..." and he would do the "That's a joke, son! Ah say, that's a joke!"
The blue bonnet hen character is a direct lift from the Milton Berle show. A rich woman would stumble into the sketch and only reply with that Yeeeeeeesssss.