might- possibility, the speaker does not know if he did or he didn't could- emphasis, the speaker knows that he was very scared but he didn't piss himself

Hey Chino, No, these words are rather different from one another. ''Could'' is sometimes used to place emphasis on a situation; whereas ''might'' is used to talk about an event that is a remote possibility. I shall be able to explain the usage of these words to you, as follows: 1) Ability in the past: If you want to say that someone had the ability to do something but did not in fact do it, you say that they ''could have'' done it. E.g.: ''You could have given it all to me.'' E.g.: ''You could have been a little more polite.'' E.g.: ''You could have won the game if you wanted to.'' 2) Possibility in the past: You use ''could have'' to say that there is a possibility that something was true in the past. E.g.: ''He could have been in the house on his own.'' *** ''Might have'' can be used in a similar way. E.g.: ''She might have found the information online.'' E.g.: ''It might have been a dead bird.'' *** You also use ''could have'' to say that there was a possibility of something being true in the past, although it was not in fact true. E.g.: ''It could have been worse.'' E.g.: ''He could have made a fortune as a lawyer.'' In regard to your sentence, I should say that it conveys the idea that the person was so scared that he was about to piss himself (he did not, though). So, in this case, ''could have'' should fit your sentence quite well as it is a better word to describe / give emphasis on how scared the person was at some time in the past. Hope that helps!!!