One possible theme could be the fear of the unknown. Space is vast and mostly unexplored, and a space snake is an unknown and terrifying entity. Another theme might be survival, as the characters in the novel would likely be fighting for their lives against this space - bound menace.
Perhaps the theme of madness. The encounter with such a horrifying space snake could drive the characters insane. The constant fear and the unnaturalness of the situation in space could lead to psychological breakdowns. There could also be a theme of discovery, as the characters might learn more about the nature of the space snake and the secrets of space through their terrifying encounters.
Sure. 'The Call of Cthulhu' by H.P. Lovecraft has a similar sense of horror. It involves otherworldly and terrifying creatures from the unknown. Another one could be 'Alien' novelization. It has a similar feeling of being in a dangerous and unknown space environment with a menacing creature like the space snake in your mentioned novel.
One possible theme could be fear. If Pete is afraid of snakes, it could be about him facing his fear.
Another theme might be the loss of humanity. Horror often shows the dark side of human nature or the struggle to hold on to one's humanity in the face of terrifying events. In this novel, perhaps the events that unfold cause the characters to question their own and others' humanity.
One possible theme is the power of the unknown. The snake is an unknown and mysterious creature, and the girl's marriage to it shows how the unknown can have a great impact on one's life.
One possible theme could be the fear of the unknown. Since the knock is something mysterious, it represents the unknown danger lurking.
In 'gorgon a horror story', a likely theme is the battle between good and evil. The Gorgon could be seen as an embodiment of evil, and the characters who oppose it represent good. Another theme could be the theme of transformation, not just in the literal sense of being turned to stone, but also in how the characters are changed by their encounters with the Gorgon. They might become more cautious, more desperate, or even more heroic in their attempts to survive.
The theme of the unknown can also be present. There could be strange, unidentifiable shapes or noises in the dream. The dreamer doesn't know what these things are or what they might do, and this uncertainty creates a sense of dread.
A theme of betrayal could also be present in the Veronica horror story. Perhaps someone she trusted, a friend or a family member, has led her into the horror situation. Or there could be a betrayal from a supernatural entity that she thought she could trust or make a deal with. This betrayal then becomes a major part of the horror that she has to endure.
Another theme might be the idea of a cursed object. The ribbon could be cursed, and anyone who interacts with it is doomed to face horror. This theme often plays on the fear of the inescapable consequences of coming into contact with something that is 'tainted'. For example, the characters in the story might try to get rid of the ribbon, but it always finds its way back to them, bringing more and more horror. It could also be about the theme of revenge. Maybe the ribbon was once a symbol of love or friendship, but something bad happened, and now it's a tool for revenge, and the horror it brings is the revenge being exacted on the unwitting victims.
Maybe in follistem horror stories, a common theme could be the loss of identity. Characters find themselves in follistem and slowly start to lose who they are, whether it's due to some strange force or entity. Also, there could be a theme of betrayal, where people trusted someone or something in follistem and were horribly let down.