First of all, I think you are a bit confused with the word "こわいい," as such a word does not exist. Perhaps you mean こわい (scary)? If this is the case, I can certainly explain differences in the forms of こわい and いい. An adjective ending in ～かった is affirmative, but it is also in the past tense. So let's make that clear as well. いい　・　こわい - affirmative, present tense よかった　・　こわかった - affirmative, past tense. The reason こわい does not become "こわよかった," just as いい becomes よかった is simple. The adjective いい is based off of a similarly sounding adjective よい. Both are said, but in the present tense you will most often say いい. よい and いい have the same meaning and are therefore interchangeable. but when you conjugate them into the past tense, they are the same word. いい is the only adjective of it's kind that has an alternate form where it's present and past forms differ so much. I hope this explanation answers your questions and that you now understand it. If you have further questions about adjective conjugations, please let me know.

If you write the part of kanji for this word 「可愛い」, you may know why the past tense is 「可愛かった」.