Is it when the comparative form of an adjective is not available?
•Conjuring 2 is more horrible than The Nun
•Conjuring 2 is creepier than The Nun
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The comparative of a two-syllable adjective ending in -y is formed by adding -ier.
easy - easier
creepy - creepier
We can say
easy - more easy
creepy - more creepy
but it's less usual.
Source: Grammarway 4 by Jenny Dooley and Virginia Evans
I remember reading somewhere that two-syllable adjectives that end in -y can have two forms for comparison, for example easy-easier/more easy; happy-happier/more happy. But I don't think it is a rule in English, the language where a most economical form is preferred. So, although it's not grammatically wrong to say "more easy/happy/ugly (etc.)", it is just not normal in English
Also, there's quite a lot of adjectives which cannot vary in intensity or grade and have a quality that can't be compared. They are called absolute adjectives, and horrible from your example is one of them (here's a list of some of those).