Okay, having listened to a whole bunch of calls from various creatures on-line, I think the Thing That Went "HNAAAGH" was a frog. I'm not sure what KIND of frog--the American Bullfrog was pretty close, and I am ashamed to admit that all this time, I thought bullfrogs made that classic "erRRRiBBit!" noise, but it is indeed a sort of flat buzzing "hnAAaaGH!" I'm not sure if that's the right kind of frog--why the supposedly nocturnal frog was awake in the hottest part of the day, calling moodily across the swamp is a mystery--but having exhausted my brain with frog calls and egret calls and cormorant calls and heron calls (and a more ungainly array of squawks you've never heard--those birds sound like Tom Waits suffering severe gastrointestinal distress) I'm willing to chalk it up to "Probably a bullfrog."
So, um...what the hell kinda frog goes "Ribbit" then?