Oh, boy. The Shrike. That thing still haunts me in a way that no other monster or alien across all of Sci-fi or fantasy really does. It's something about the inscrutability of it, especially in the first novel (still my favorite) where its purpose and backstory haven't been revealed. Sure, it's scary, but I think the mystery of its motives - and its ability to unpredictably act apparently benevolently sometimes - is where the real terror lies.