Behemoth

Behemoths are "the largest space faring creatures in the chartered galaxy." Immense and docile creatures and bearing resembelence to manta rays, behemoths travel through space by using their own energy, presumably as part of their metabolism. Given the speed at which they can travel, they are seemingly able to harness enough energy to enter warp space.

Behemoths were incorporated into the Zerg gene pool when they passed close to Zerus, lured in by the psychic beacon of the Overmind. This gave the Zerg the ability to travel through the vacuum of space, allowing them to leave their homeworld. Although the Overmind could create its own gateways into warp space, the behemoths seemingly still remained in use, given that, due to their immense size, they could act as organic assault carriers, able to carry even Overlords in the folds and pockets of their sprawling bodies.