An unnamed Delta Quadrant species for whom the dream state is true reality, and who attack the waking races they fear by trapping them in a shared dream.
The Dream Species are never named. They appear in the Voyager episode Waking Moments as heavy-browed humanoids who spend their existence asleep, their bodies lying in a cavern while their minds meet in a collective dream. They look on waking species as a threat.
Several of Voyager's crew dream of the same alien, and some fail to wake. Chakotay uses lucid dreaming, with the image of Earth's moon as his signal that he is asleep, to enter the shared dream. The aliens appear to seize the ship, and it emerges that the whole crew is asleep and dreaming a common illusion.
Chakotay, the only one to wake, finds the sleepers on a nearby planet. Fighting off sleep, he threatens to have the Doctor destroy their cavern unless the neurogenic field is shut down, and the crew are released.