Species 8472
The real name for this species is not known, so the Borg name for them is used instead. The first recorded encounter by this species took place about five months before Stardate 50948.3, and it is assumed that the Borg intruded upon the dimension in which Species 8472 existed. This provoked a massive attack by Species 8472, and one the Borg was not able to handle. This time the enemy proved more powerful than themselves.
Species 8472 uses biological ships, which seem to be made of the same organic matter they themselves are made of. This organic matter is also immune to the Borg nanoprobes due to a highly aggressive and effective immune system, making them impossible for the Borg to assimilate. Everything that penetrates the organic matter is immediately destroyed.
The native domain of Species 8472 is a dimension totally unlike our own, and instead of the vastness of space, a fluidic matter exists everywhere - not unlike an ocean. Their dimension can be entered by opening a wormhole by creating an artificial singularity.
The EMH Doctor aboard the U.S.S. Voyager found a way to reprogram the nanoprobes the Borg uses for their assimilation to attack and eliminate Species 8472 with. This biological "weapon" the crew aboard the U.S.S. Voyager traded to the Borg with a free passage and escort through their space in their attempt to get home to the Alpha Quadrant.
Species 8472 is also somewhat telepathic, since they several times made "contact" with the now departed Ocampa crew member of the U.S.S. Voyager, Kes.