This happened to me one day at my grandma's home around 2001 or 2002. My little brother and I were messing around in her backyard. When we were looking under rocks, we found many weird worm-like creatures that would inhale loudly, then breathed out without a sound, then looked dead after the long exhale.
Then when we saw another that looked the same, but this time we could see in the mouth was filled with little teeth, and it smelled funny.
Our grandma was out shopping, so we couldn't tell her. We kept it to ourselves. But there were at least four of the "worms". I don't remember every detail, but they were pickish gray and had no eyes that we could see. The one that moved kind of just stayed still and squirmed; it was weird, but they all went away the next morning. There was medium-sized hole near a rock close to her fence. Did they leave?