As a note: none of these stories should be taken as any sort of proof of alien life, anywhere, but goodness knows humans will always have their urban legends... or space legends as the case may be...
On some far flung colony, somewhere, the terraforming went horribly wrong. The first wave of colonists to settle there were slowly and irrevocably poisoned by the food they ate and the water they drank. Slowly, their brains stopped producing the chemicals needed to operate properly, dampening their intelligence and making it imperative that they find a new source of those chemicals or else have all brain activity cease. The colonists found the answer in eating the brains of various animals that lived about, but as the chemicals in these animal’s brains were not sufficient to replace the chemical’s lost, the colonists survive only at a very base level of intelligence, craving what their bodies no longer produce.
It is said that any ship unlucky enough to land there inadvertently sets off a frenzy amongst the colonists, as they are instinctively drawn to harvest the pure source of brain chemicals present in the minds of the ship’s crew…
It is said that in the darkest reaches of space there are creatures that survive and thrive in the zero-gee environment. Some say they look like whales, some like jellyfish, but most agree that they swim the solar winds gathering minute particles of matter as their food. Generally considered to be benign, these creatures are nonetheless huge, easily dwarfing most Isley ships. Those who believe such creatures exist claim that the only reason that no-one has ever been able to reliably claim to have seen one is because they only frequent the darkest spaces between the stars, and space travel has only breached such areas with the advent of hypersleep 50 years ago. Obviously, if everyone is asleep when ships pass through such areas, no one is going to be able to take a picture...
Alternately, Space Dragons.
A lot like space whales, only more likely to eat your ship.
Chupacabras in Space
Aside from the local ‘Nessie’ or ‘Big foot’ legend which exists on nearly every habitable planet, one colony claims to have actually imported their own cryptozological wonder from Terra. For nearly a hundred years the Colony of Los Santos, a smaller colony situated on Callisto, one of Jupiter’s moons, has had stories about this ‘animal’ which was once believed to inhabit the Terran Island of Cuba. Local lore claims that as urban sprawl choked out the last of the countryside on the island, the last of the Chupacabras cleverly hid amongst an animal transport to the new colony. Once there, the Chupababra thrived, but is still too wiley to be caught, or seen, by humans. It’s favorite meal? To steal gen-grown animal protein right out of the extensive grow houses on the moon.
I saw the Icarus!
About every five or ten years someone somewhere claims to have seen a derelict hulk of a ship floating through the galaxy, which they claim is the long-lost Icarus. Usually, such claims are ignored, as the first several ‘rescue missions’ sent out found only empty space, or more recently, wreckage from the colonial skirmishes. However this doesn’t keep people from claiming to have seen the ship ghosting its way back into the galaxy, claiming that the Icarus is either returning from a successful mission, or that its ravaged and empty hull is drifting back into the milky-way on the solar winds. It is a favorite topic of the premier gossip rag of the day, the Mars Enquirer.