Roller

The "Rollers" (as they've been nicknamed by Bob Carson); also known to Humans in general as the "Outsiders"; are a spacefaring hostile species of spherical life forms. Their origins aren't known, except that they hail from an unknown galaxy in the general direction of the Pleiades. As an intrinsically hostile species; peaceful coexistence between different races is not part of their normal psyche.

Biology
Their bodies are almost spherical in shape, red in coloration and about a yard in diameter. As their nickname indicates, they move around by rolling, which is accomplished by somehow shifting the gravitational center of their bodies.

While it has no actual limbs and nothing that resembles a mouth, eyes or other sense organs, a Roller's body does have a number of grooves from which retractable tentacles can emerge. Each tentacle is estimated at being around an inch in diameter and 1.5 ft. in length. These tentacles end in bifurcated tips, forming what essentially amounts to fingers, which in turn possess hardened tips, similar to small claws or nails. These aren't actually more dangerous than Human nails, and overall, the physical strength of the tentacles is rather unimpressive. However, what the Rollers lack in offensive capabilities is made up in their defensive ones: their bodies are protected by a tough hide which isn't easy to penetrate.

Members of this species are known to possess at least a rudimentary form of telepathy, being capable of transmitting abstract thoughts and ideas even to non-telepathic life forms, like Humans. Whether or not this kind of mental communication is more refined when used among the Rollers themselves is not known, although it seems likely.

Culture and society
Rollers are an expansionist spacefaring culture, and seem to operate on raging xenophobia. Telepathic communication with a representative of the species has revealed that they have no interest in peace or coexistence, and harbor nothing but hatred towards species that they view as inimical, such as Humans.

Technology
Roller ships tend to have slightly less firepower than similarly-sized Human ships. In compensation, the Roller ships tend to have superior speed and maneuverability, thus making both factions more or less equally matched in battle.

History
At some point in history, Roller ships started raiding and destroying Earth colonies at the fringe of the solar system. As the conflict escalated, the battles always resulted in complete annihilation of one side and thus, Humans were completely unaware of what their adversaries looked like and why they were attacking. They referred to the Roller fleet as the "Outsiders", and knew only that they originated from another galaxy in the general direction of the Pleiades.

Just as both races where already mobilizing vast fleets, ready for the first large scale battle, a non-corporeal alien intelligence interfered by abducting a Human soldier named Bob Carson and a Roller soldier. The alien brought both of them to a desert landscape outside of regular time and space and explained that both of their races had potential to evolve, but that the ensuing war would result in the annihilation of one and severe crippling of the other, thus holding back the natural development of both. To prevent that outcome, the alien decided to end the conflict in a way that would preserve one of the two races, by eliminating the other from existence. Carson and the Roller would thus fight each other to define which side would be killed and which would be spared.

After some futile attempts to propose mutual coexistence and peace, Carson fought the alien and won, resulting in the annihilation of the Roller species and the survival of humanity.

Appearances

 * "Arena", by Fredric Brown (1944)