I think 'most horrible' is grammatically correct and 'horriblest' , although strictly speaking is grammatically incorrect, is used colloquially. Usually you can add 'est' to an adjective in English to make the superlative eg. big, biggest, small, smallest, but horrible is an exception to the rule. So it depends on how you would like to use the word and the context, maybe in formal writing you would use 'most horrible' but if chatting with friends you could use 'horriblest'.

Rule of thumb: Use "-er"/"-est" for adjectives with one syllable (small, fast, cheap) or ones with two syllables that end in -y (busy, lucky, happy), otherwise "more"/"most". So: "most horrible". – https://www.ef.com/wwen/english-resources/english-grammar/comparative-and-superlative/ – https://www.onestopenglish.com/support-for-teaching-grammar/comparative-and-superlative-adjectives-article/144846.article – https://www.grammar.cl/Intermediate/Comparatives_Superlatives.htm