The breeze was picking up, which made two noticeable changes.
First flowing tip of the structure began to wave more wildly, and the three figured standing at the edge of the bridge could now see that the entire thing was covered in similar, though smaller undulating forms. For all the world it looked like some sort of underwater plant structure being moved by the current.
Second, whatever was following them was emitting some sort of gas, or else kicking a significant enough amount of dust into the air that it could be seen, in little puffing wisps, as the wind swirled around the horses hooves. The animals danced where they stood, nickering and nervous.
To one side, about twenty feet from the party of adventurers something pushed or pulled itself up out from between the flagstones. It was just a smudge at first, but after a moment a little hand could be seen, webbed and nearly black, then a head and a second hand. A glance behind or two the other side would reveal more, similar but not identical creatures. They weren't large, about the size of a large house cat, but all four of their limbs gleamed sharpy and their mouths were nothing but spiking rows of teeth. The first one to fully pull his form, from an impossibly small crack in the road hissed quietly, ears rotating for information but eyes locked on the party. The gas or dust that had been seen was thicker now and seemed to come from the little creatures; not from their mouths but just puffing out of them as they moved. In the moment after the first hiss a dozen more sounded, coming from every direction except the bridge.
Suddenly a rat, obviously realizing that this was not the place to be, scuttled out from a pile of discarded clothing and tried to run for the nearest building. Half way he ran through one of the puffs of gas and began to convulse, twitching and foaming at the mouth until the little body simply split from chin to tail along his belly, turned half inside out and died.