(amorphous spherical body)
When pregnant with twins, one side becomes deformed for unknown reasons.
It is unable to survive on its own as it has no bones or major organs.
As long as the umbilical cord is connected, it grows and grows.
Most of them are spherical, and as you can see from the description, if you split them open, the inside is full of flesh and internal organs.
Usually comes from domesticated animals.
It is said that it sometimes occurs in humans as well.
The photo is a cat in a box.