Space drifter

This is one of the biggest creatures I ever designed. It lives not on a planet but in the void of space. The hardest environment posible for life. It is also not affected much by gravity. So it has reasons to and can grow GIANT.

Space drifters usually grow to size of 4 kilometers long, but some reach length of even 10 kilometers. They feed by photosynthesis when orbiting close to star, but cannot find much water and carbon dioxide they need for this proces in star life zone. So they must "dive" far away from the star into cold darkness, where more comets can be found. Space drifters attach themselves to such comet (or a passing spaceship if there is any around) and eat frozen water, carbon dioxide and some other useful components. Then they return closer to star, where it is warm enough to process and digest their prize.

Their body is organised in plectenchyme, the type of tissue found in mushroom fruiting bodies. Thats mean it is colossal entanglement of thin vine-like tubes. Unlike mushroom hovewer, these vines can change their length, and so they can act like muscles. Entire body od space drifter can not only move, but also change its shape. When it would be close to star it would expand fins on its sides to collect light and heat, so they would look like giant sails. When collecting nutrients in darkness it would expand tentacles on its front. Their egg shaped "tail" serves as propulsion, biological version of nuclear engine is located here. It doesnt have any brain, but network of nervous cells in its body is gigantic. When prowoked and forced to invest its precious energy into thinking process it can act quite intelligent.
Space drifter reproduce by buding its offspring into middle section of its body. When it grows big enough to survive by itself, body of its parent opens and releases baby giant. Wound in adult specimen of course heals quicly later, shapeshifting is very usefull ability!

Space drifters inhabit about 20 star systems in Milky Way Galaxy (close to each other) but long way from Earth. They are able to cross distance between two star systems, but as they cannot travel faster than light, it is very dangerous, hundreds or even thousand years long journey, without any posibility to feed during it. Probably only small fraction of space drifters brave enough to set out on such journey survive it.

But how such gigantic and quite organized life form could evolve to inhabit void as single-species ecostystem? Well most acknowledged theory suggest its ancestor was probably some kind of biological spaceship or Von Neuman probe, which gone feral many million years ago and then evolved into these monsters.