He asked: Who came up with the word "dingleberry"?

My answer: since you asked....dingleberry is a slang term for the unfortunate condition of fecal matter being caught in the hair or fur of an animal or human around the region of the rump or buttocks. it generally occurs when the afflicted are unable to wipe or remove excrement from the nether regions and the matter dries. they occur on animals, especially those with thick coats or with people who have the inability to wield toilet paper in an appropriate manner. animals such as long-haired cats, dogs and sheep may have to have the offending berries removed by trimming hair to prevent other nasty things occurring such as fly problems. humans (mostly male as they are the more hairy) can wax or shave the hair or learn to use toilet paper properly. another piece of advice is to use expensive toilet paper as it does not disintegrate and leave behind debris that may become dingleberries. using a bidet may also help. (hey....I have 2 in my house!)
oh wait.....you asked about the word origin!....well..the word is not featured in the Oxford nor Chambers dictionaries but the word dingle is, and may give some clue as to the word's origins.
there is also a Dingleberry Lake in California. odd but true.
and now----I bet he is sorry he asked!