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There is a specific and terrifying difference between “never were” monsters and “are not anymore” monsters
“The thing that was not a deer” implies a creature which mimics a deer but imperfectly and the details which are wrong are what makes it terrifying
“The thing that was not a deer anymore” on the other hand implies a thing that USED to be a deer before it was somehow mutated, possessed, parasitically controlled or reanimated improperly and what makes THAT terrifying is the details that are still right and recognizable poking out of all the wrong and horrible malformations.