Help me make monsters scary again

So I'm busily populating my Points of Light hex map with encounters and threats, and I'm having a really hard time with monsters. Gryphons and harpies and werebears...they're all so quintessentially fantasy, but they don't scare me -- and I worry they'll have a similar effect on my players.

Oh sure, they're threatening and all. They can maim and kill characters, and in a pinch the PCs can rally together and bring 'em down in a hail of arrows, lightning bolts and fancy swordplay. But they're not scary, not in the sense that they inspire the PCs to do much more than simply slay them. And it ain't like I can't pull off a terrifying scene setup -- I can don't worry 'bout that. But I see more storytelling potential in "monsters" that stand on two legs: corrupt nobles, death cults, warmongering orc raiders, nefarious highwaymen, etc.

I think it's because monsters aren't smart. With a few notable exceptions, they don't plot or scheme or try to worm their way into positions of power. They react to stuff and defend themselves if attacked. That's about it. I've read James Maliszewski's treatise on Gygaxian Naturalism, and it's a very cool way to think about monsters -- but that still doesn't solve their innate lack of smarts.

Anyone have any suggestions about how to plop a few full-on monsters in my map -- without having them simply become speed bumps with hit points? Or, a better question might be: what are some monsters that are genuinely intelligent and could offer a real, perennial threat to the campaign?