I read this a long time ago - perhaps online in a newsgroup, in the 90s, perhaps in a printed collection.
What I remember is this guy was driving through the desert on a highway. Some roadside trash was blown up onto his windshield, and there was a mouth on the bottom side. Apparently these creatures/aliens/whatevers imitated roadside trash - flattened cardboard box, etc - and would eat the roadkill animals.