No, it is not a correct English word.
Apparently Merriam-Webster includes an adjective definition for 'horror', but I have to disagree with them here.
The example usage is 'horror movie', which you've also included in your question.
However, 'horror movie' is a compound noun , and in that phrase, 'horror' is acting as a noun adjunct , not an adjective.
A noun adjunct is a noun that is used in a manner similar to an adjective.
This does not automatically result in comparative and superlative forms, such as 'horrorest' and 'horrorer'.
Near the end of the linked 'compound noun' article, there's a discussion on alternative forms in natural language.
The example compares using a noun adjunct versus an adjective based on the noun (an inflection).
In this case those two forms would be: Noun-Adjunct
A horror movie
Inflection
A
horrifying movie
Inflection Comparative and Superlative A more horrifying movie
The most horrifying movie
ell.stackexchange.com/users/10238/DCShannon
Horror is not an adjective.
It is a noun.
In the sentence "I saw a horror movie yesterday", it seems like an adjective, since it modifies "movie", but that's not what's actually happens. "
Horror movie" is a compound noun.
"Horror" can also be a noun by itself
Hey, could you recommend some good horror for me?
You could also substitute another movie genre for horror in the above sentence.
Action, adventure, comedy, drama etc.
and it works the same way.
Adjectives similar to horror are Scary, creepy, unnerving, unsettling, disturbing, frightening, horrific, terrifying...
ell.stackexchange.com/users/15406/James
You've asked two questions in one.
First, horror cannot be used with the -er/-est comparative suffixes; native speakers would only ever use more and most .
This is simply because horror has more than one syllable .
There are a whole bunch of exceptions both ways (see the discussion in the comments and this more thorough explanation on Wikipedia ) but
the basic principle is that the comparative suffixes are primarily used with single-syllable words of Germanic derivation, while more and most are primarily used with many-syllable words of Latin derivation.
Second, more horror is not how a native speaker would compare the grade of horror in two works of fiction -- this movie didn't scare you much at all, that movie scared you a lot more.
Instead, we would say one was more horrifying than the other.
And one particular movie might be the most horrifying movie you have ever seen.
The logic behind this word choice is: you're describing something the movie did to you (it induced the emotion of horror) and you're comparing how effectively it did that .
The movie did something, so that requires a verb, specifically the -ify verb form of the induced state.
Then you convert the verb back into an adjective with -ing to make it an intrinsic quality of the movie, and now it can be compared to the same quality in other movies.
(We still can't use -er/-est , because horrifying has even more syllables ... except that *horrifyingest has so many stacked suffixes that I can imagine someone intentionally using it, for effect .)
More horror is also something native speakers might say, but it means something different and is used with different main verbs.
If I say movie A has or contains more horror than movie B, that means more of the time of movie A is spent on storytelling elements that are typical of the horror genre; this might or might not correlate with movie A being more horrifying
(perhaps A has so much horror in it that it goes over the top (sense 2) and becomes ridiculous).
(Boldface: emphasis.
Italics: mention, not use .
Leading asterisk:
marks descriptively-incorrect construct.)
( More horrorshow means something completely different .)
ell.stackexchange.com/users/2823/zwol
When you look it up in MW Learner's Dictionary it can be defined as an adjective: 2 horror /ˈhorɚ/adjective always used before a noun :intended to cause feelings of fear or horror but there are no comparative/superlative forms offered.
When you look up another adjective (let's say 'clear'), comparative and superlative forms are shown before the definition.
This may be used as a guide to determine whether an adjective is gradable or non-gradable .
Horror, I would say, is non-gradable, same as medical or dead .
P.S. ODO, CDO and LDOCE don't define of horror as an adjective, only as a noun.
I have found it defined as an adjective only at MW.
ell.stackexchange.com/users/18606/Lucky
Horror is a genre of fiction; it is not something you can apply to anything that is bad.
For that we have the words "horrible" and, much less commonly and a little more specific, "horrific".
Since each of those has three syllables, you would have to use them like this: horrible more horrible most horrible The best way to apply this to a horror movie would be to kind of get around it and say something like "the movie that is the most focused on being a horror film". ...
As mentioned in the comments, "horror", "horrible", "horrific", and "horrifying" all have different meanings, with "horrific" and "horrifying" being particularly close in definition.
ell.stackexchange.com/users/4427/Panzercrisis
I know that "horror" is also an adjective as in  I saw a horror movie yesterday.  But I don't know if its comparative and superlative forms are "horrorer" and "horrorest" respectively. I searched on Google and found that these forms are used  by people (mostly Indians) who are not quite good at English. I didn't find any standard source that uses these forms of the word. Please let me know if these forms are correct and accepted in standard English or not.
Is "horrorest" a correct English word?
ell.stackexchange.com
<adjectives><superlatives>
ell.stackexchange.com/questions/54809
ell.stackexchange.com/users/15866/Gurpreet
No, this is not an English word. When you look horror up in  MW Learner's Dictionary  it can be defined as an adjective:   2 horror /ˈhorɚ/adjective  always used before a noun  :intended to cause feelings of fear or horror, but there are no comparative/superlative forms offered. Horror cannot be rendered as a comparative adjective, thus 'horrorest' is not a word.
Horror cannot be rendered as a comparative adjective, thus 'horrorest' is not a word.