They don't have to be no treasure
There's ways to do things like this without inexplicably giving the monsters wealth. Here's a few:
Location, Location, Location
In the module Ruins of Myth Drannor for 3.0 there's a small dungeon sequence in which almost none of the enemies have treasure. Thankfully, the dungeon itself is full of valuables! Consider having the monsters guarding, or perhaps maintaining lairs, near non-gold valuables that can be converted to wealth - such as, for example, gems, paintings, rare materials (such as ironwood), expensive tools, etc.
Supply and Demand
Maybe the creatures themselves can be valuable! A gourmet chef nearby has a bounty on the beasts - he's experimenting with a new recipe. Maybe the local military wants someone to turn in hides, ears, etc as proof that they're fighting an infestation and will pay well to do so. A local wizard wants live specimens for his experiments, or other things of that nature.
Increased Wealth Later
Do these creatures serve a master? Perhaps he's trained them to bring valuables to him and as a result has higher wealth than his CR might suggest. Your players could even encounter some of the creatures en route, still carrying valuables that they can loot. Alternately, grateful townspeople could offer them rewards or discounts on their goods that could simulate higher wealth through lower prices.