Well in the session i'm prepping this week, there's a giant carnivorous bramble plant that's pretty hard to spot until you're already knee-deep in it. It's completely inert until you scratch yourself on its thorns. Once it's tasted your blood, though, it will grab you and drag you off to its heart, where it slowly drains you dry.

It makes traveling though the forest a very deadly proposition for anyone who doesn't know how to spot them.