Borg: A very hostile species that travels in massive cubes and spheres. (Hostile actions toward this species is not recommended!)
Romulans: A highly sophisticated species whose logic is to destroy anything they can.
Species 8472: A very hostile three legged species that is so powerful, it intimidates the Borg! [Do not offend!]
Cardassians: A warlike spices that can pose a major threat.
Kazon: This spices travels in large ships that, if attacked correctly, can be taken.