Aliens from fluidic space who fought the Borg.
Species 8472 is a species of fluidic organisms who live in a realm known as fluidic space, a dimension that stands completely apart from the normal universe. They are powerful, strong and extremely advanced, and their bio-ships are very hard to defeat. They are largely unknown to the rest of the galaxy.
Their biology allows them to survive in conditions that would be deadly for most other life forms. Their ships are alive, and the creatures themselves have a tough and adaptable body and are unusually resistant to many weapons. They are at war with the Borg, who had tried to assimilate them and found them to be their equals.
They first appear in the Voyager episode Scorpion, in which the Borg are losing a war against them. Captain Janeway makes a deal with the Borg for help against Species 8472, with Seven of Nine being drawn into the effort, and Voyager's crew eventually finds a way to modify the nanoprobes to be effective against them. A later episode, In the Flesh, shows them building a copy of Starfleet Command as part of an infiltration plot.