Castor is different from all of the known species in the Milky Way.
Castor is a highly advanced civilization composed of a single organism that divides and replicates itself into many forms.
Castor can present itself in the form of an individual creature or a horde of them. It can assume the shape of a single human being or an assembly of them. Each of these manifestations shares the same consciousness, that of one Castor.
Castor can reject its solid state of matter and dissipate into a fog, a fog that may become rain and evolve into a storm. Additionally, Castor can simply exist as an entity that dwells outside our material realm but is fully aware of us.
There is no known origin for Castor; they are believed to be immortal. The architecture of their habitats is so advanced that we humans can hardly make sense of it.
One could almost think that Castor has reached perfection, but something seems to be missing. They can create life outside of themselves and are obsessed with bioengineering a perfect new species by calibrating a mix of DNA samples collected all over the galaxy. What Castor doesn’t know is that adding human DNA to the mix may create a creature so vicious and invincible that it could result in the extinction of 90% of intelligent life in the Milky Way, possibly exterminating Castor as well.
Artist representation: Castor as a Giant Moth-like Alien