I think the intelligence behind the flashback was just “possessing” Lyn to make sure they were paying attention. It will probably “possess” others now that Lyn will be out of the picture.
You must be new. If a person is conscious and mutates, they turn into “bloodthirsty monstrosities.” If they are unconscious, like Holly was, they become “animal-like mutations.” The fetus was essential unconscious, so it became a fish. If the person is on drugs, they become something really weird.
From Wiki: “The Coco (or Cuco, Coca, Cuca, Cucuy) is a mythical ghost-monster, equivalent to the bogeyman, found in many Hispanic and Lusophone countries. He can also be considered a Hispanic version of a bugbear, as it is a commonly used figure of speech representing an irrational or exaggerated fear.”
You can still get that feeling by going to your local library and checking out the dvd section. And better still, it doesn’t cost money!