Rosemary's Baby (1976)
by Klaus Poppe
The story was told in the first person through the eyes of a young boy.
His mother was pregnant with twins,
and for some reason he was worried that a monster would be born instead.
In the end, the boy himself was the monster.
But Klaus Poppe’s weirdness came in when the boy feels relief
at finding out that he is the monster,
and ends up loving his little brother and sister like a normal sibling.