Ha. Fair question.
It was gone a month later. The IntMed doc that I txfr'd to (a boarded internalist) stuck with "No idea." They talked to the owners about surgical explore (when we first saw the cyst), but even at that point the dog was essentially back to normal (bouncing off the walls puppy), so they decided not to explore and do the recheck in a month instead. By then it was gone.
Most likely a pancreatic pseudocyst (differentials would be mass or abscess). You see them with pancreatitis sometimes, and if the patient recovers normally the pseudocyst goes away within a few months. What is weird is the age - that doesn't fit well with pancreatitis.
I dunno. Maybe the dog had some viral gastroenteritis and the inflammation was enough to piss off the pancreas and it developed a pseudocyst. Could speculate all day, but in the end - dunno.