A horde of zombies is nipping at your heels, if one of the lurching horrors chomps down you’re doomed to a horrifying death? To me, that invokes fear.
One Siberian tiger, or a pack of velociraptors, is deliberately picking your party off, one at a time, despite the fact that humans are lousy prey animals, just because they can? That invokes terror.
I think both have their place in scaring the hero – and reader – silly. And they can both be good antagonists, depending on the book. P.M. Griffin’s Star Commandos series has plenty of non-intelligent swarm monsters to make your skin crawl; the heroes facing off with the ravagers in the first book is more than enough monster for any soul. (They also have to deal with some slimy bad guys, but next to the ravager swarm – not so much a problem.)
Still, for a fantasy adventure setting, I think I prefer a few intelligent monsters breathing down the main characters’ necks. In part it might be due to the tools available to heroes. If your wizard can cast fireball, a swarm of army ants is crispy critters. But if you’re up against a dragon, or even a magically enhanced tiger? Those creatures figure out pretty darn quick that the guys in funny robes with wands are dangerous, and change their tactics accordingly. Meaning they’ll wait and strike your archmage down in the water closet.
(No, seriously, tigers have done this to hunters in RL. They think.)
Intelligent monsters mean your hero can’t just win by main force. They have to be smarter than the monster. Not to mention a bit lucky.
So… I think I prefer antagonists that let the heroes show off what they’re made of – brains as well as brawn. Granted, you can do that with swarms, too. It’s just that they don’t change their tactics very fast, so the hero doesn’t get to show escalating levels of improvisation and thinking fast as easily.
For swarm, the ravagers still are among my favorite, partly because they did have a hive mind. For intelligent – the original Predator movie is still Very cool.
Preferences? Thoughts? Favorite monsters – swarm or intelligent?