In a swarm monster, multiple enemies are represented as a single monster mechanically. This is practical because it is easier to manage for the referee and also interesting on the player interface side because a swarm of flying demonic bells might be immune to most weapons but vulnerable to area effects and perhaps sweet singing. (I believe 3E should get credit for this innovation, though I am not sure about that.)
In a recent post, Gus catalogued a number of ways to make solo beasts more interesting and challenging. To this toolbox, I would add the inverse swarm, which is a single enemy represented mechanically by multiple monsters.
So an elder dragon might have head, body, two claws, wings, and a tail, each with a separate attack, different ACs, and its own HP total. This avoids the biggest weakness of beasts versus adventuring parties, which is the limited number of actions the monster can take (1, or maybe 3 for a claw claw bite routine) compared to the 6+ chances an adventuring party gets to take on the attack roulette wheel each round. It also allows interesting strategies, like disabling particular abilities.
The only real downside is that the referee needs to spend some thought on the monster, preferably beforehand (though it is possible to improvise a less complicated inverse swarm).