An unnamed and intensely territorial Delta Quadrant species which defends its space with thousands of small ships acting together, encountered by Voyager while taking a short cut in 2373.
The Swarm species are never named. They appear in the Voyager episode The Swarm, where they are known for attacking any vessel that crosses their border, and for a language so alien that the universal translator can barely handle it.
Going around their territory would add about fifteen months to Voyager's journey, so Janeway decides to slip through its narrowest part. The crew find a ship whose crew have been killed, and learn how the aliens fight: their small vessels attach themselves to a hull in vast numbers and drain its energy, linked by an interferometric lattice.
When the swarm descends on Voyager and boarders reach the bridge, the crew disrupt the lattice so that the destruction of one ship sets off a chain reaction among the rest, and the survivors withdraw.