Whip-weeds are small green animals that have the forms of green plants. A colony of them appear as a carpet of harmless green plants, other than that sway slowly from side to side, even when there is no breeze
Once an unsuspecting victim steps on the center of the patch, it erupts about them, pulling their "roots" from the ground, and swarm all over them with their poison-filled stingers.
One such colony was found in a tunnel in the Cavern of Doom, before a chest of jewels.