SPECIES 8472

As Kes notices at the end of season 3, "It's not the Borg we should be worried about... it's them.". "Them" refers to Species 8427.

Their favorite slogan:" The weak will parish".
They originate from another dimension entirely called fluedic space, and entered Borg space through an artificial wormhole. They are unlike anything ever encountered; their ships are in fact organic, with "veins" carrying plasma fluid, and sections "blinking" with neurochemical pulses.

They truly are undefeatable, and the Borg have been thrashed by them for over five months.