Many feel this is one of the dumbest monsters ever, with good reason. All I think one really has to do to make them effective is shrink their size and increase their numbers.
Use the stats for a wild dog unless otherwise noted.
Number encountered: 5d4
Acid: Digesters can spit acid up to 15’ away (+3 to hit). They have no other attacks. The acid does 1d4 damage upon impact and 1d3 per round thereafter (for 1d6 rounds) unless doused with water (or some other solvent). They never appear to run out.
These two-legged beasties are shy and skittish at first, approaching adventurers warily. Once they have determined that a creature is edible, they surround it and begin spitting acid to pre-digest it. Once 1/3 to ½ their number is slain, they will retreat… but will follow at a distance, waiting for an opportune moment to strike again or feast on the remains of their former enemies. This is a good example of an 'attrition monster', one that is easy enough to dispatch at first but remains a thorn in your side (and forces you to use up resources like water, which may be a problem).
If you want a sense of the effect these little critters ought to have, check out that scene at the beginning of Jurassic Park 2 (couldn’t find a clip) where the little girl is surrounded by the tiny dinosaurs, and then imagine all of them spitting little gobs of acid everywhere.