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.
hey I totally fucked up and forgot the 3rd type, which is “Is Not Anymore And Maybe Never Was” monsters
“The thing which was no longer a deer and maybe never was” implies a creature that, at first glance, completely appears to be a deer, but over time degrades very slowly until you realize (probably too late) that it is not a deer anymore, and had you seen it in this state first, you wouldn’t have recognized it as a deer at all, and there’s a decent chance that it was never actually a deer to begin with but only a very good mimic, and what makes this one scary is the slow change from everything being right to everything being wrong, happening slowly enough that you don’t even notice it until its too late, as well as the fact that something now so clearly not a deer could have fooled you to begin with.