According to the Looney Tunes Golden Collection Vol 2 this is what happened behind the scenes:
Clampett: What happens now?
Avery: I don't know.
Clampett: Can I do whatever I want?
And Clampett did whatever he wanted, the outcome...
Daffy (To Porky):Don't let that bother ya skipper, I'm just a crazy darn fool duck! Woo-hoo woo-hoo woo-hoo woo-hoo!!
So, really Avery created Daffy, while Clampett added his "Woo-hoo woo-hoo" and his personality.