A Species 8472 bioship
The bioship was a lifeform that was used by Species 8472 as starship in the 24th century and had a similar genetic makeup to their pilots. The bioship, and Species 8472 themeselves, were resistant to assimilation by the Borg. When the Borg entered fluidic space, they were forced back by this resistance. (VOY: "Scorpion, Part I", "Scorpion, Part II", "Prey")