The poison pool is at the southern part of the Rotten Forest map. It spawns a Husk when a Barracks|soldier, reinforcement or Sylvan Elf is killed, and a Swamp Thing when an Earth Elemental is killed. Reanimated enemies are treated like their regular counterparts, but give no gold.
In the Fungal Forest, there are two poison pools, one near the center and the other near the southeastern corner. Whenever an enemy spawns from a Poison Pool, the exact one where it emerges is determined randomly.
Due to the Poison Pool's presence, it's not recommended to use Earth Elementals, as Swamp Things are very hard to defeat with their high health, fast health regen, and ranged area attacks, but give no gold; as a result, they are EXTREMELY obnoxious. Along this note, although it is strongly advised not to build Barracks due to the great hordes that will overwhelm and kill any soldier, Reinforcements can still be deployed normally after the main defense has been set up, as Husks are weak and easily obliberated by high-level towers.