The flytraps are created by the elves in Overlord: Dark Legend, and to protect the elven woods they also attacked Farmstead later in the game. Near the place where they are created you will find the Greens hive.
They usually attack in groups of 3 or 4 by biting your horde as a melee attack. The other variation of flytrap can shoot green plant food at your minions, similar to the lotus plants.
Elves can also revive ’dead’ flytraps, so they must be isolated before attacking in groups with elves.
-it’s likely that all the flytraps went extinct after the first overlord caused the war between the elves and dwarves.