How would you stat the balloon dogs?
Well the balloon dog in the movie never actually attack anything. All it did was bark and follow a scent trail.
So the balloon animal could have been a single-use "track creature" magic item. It just needs a land speed, scent and a hefty modifier to Survival (track by scent). Indeed, maybe there should be a series of
balloons of klowning that are a combination of
figurines of wondrous power and
feather tokens.
If I remember the movie correctly, there were other types of balloon too - I'm sure there were capture balloons to hold victims and I
think there might have been some kind of deadly acid-spraying balloon (although I suspect I might be imagining that one).
Should stats be necessary just give a balloon dog the stats of a Dog or Tiny Animated Object with a "balloon" special quality that makes sharp weapons lethal to them.