Animals. In many games, the most basic foe. (Heck, in World of Warcraft it doesn’t matter what level you get to, there are mutated demon boars of EXACTLY YOUR LEVEL to kill!) But the real world is often more hardcore than your average fantasy realm. Take this new fun Cracked article on The Six Deadliest Creatures (That Can Fit In Your Shoe).
There’s the golden dart frog, which is so poisonous that Wikipedia claims chickens and dogs have died from contact with a paper towel on which one had walked. Now that’s an unfortunate trap for the party ranger; poisonous tracks!
Or the scorpion which is called, with no exaggeration, the Deathstalker. Lame overblown D&D 4e name? No, it’s a couple inches long and you’re quite likely to die of it. Imagine the giant variety!
And why has poison become so lame in D&D? It’s just a DC check for some stat damage. The Brazilian Wandering Spider’s venom not only causes pain and spasms, but gives you a huge rock-hard boner like a critical Viagra overdose while you go out (no, really!). Tell THAT to your next poisoned PC and they won’t be so blase about it any more.
Or, for more inspiration, 6 Endangered Species That Aren’t Endangered Enough. Here’s a ready-made scenario, the Slavemaker Ant! It fakes death to be carried into another ant nest, where it kills the queen, coats itself in her parts, and then the locals raise the eggs it raises! You could probably do the same thing with halflings.