Life evolves on Planets and Gas Giants, so space animals would be similar to animals on Earth or the evolutions we find on Gas-Giants that have a life evolution ecosystem over a period of time such as ours. Because oceans should be able to grow as planets grow, our Galaxy has massive stars trillions the times the size of ours, which would have massive planets that could sustain liquid, so it would only take a single planet evolving life forms on it at that size to produce whales capable of floating in space that then spread out from the planet outward into the galaxy, and having planets and gas-giants the size of those in our solar system would start to rotate around it. The first whales a long time ago would have been mortal beings, but with the introduction of advanced civilizations whales would have become the main mount for space, as horses or elephants are the main mounts on Earth. Whales are easily trainable, at that size you could build anything around and in it, it would be so massive solar-systems would swirl around it so all life forms could live on it as it travelled between Galaxies or Universe Galaxies,