After extensive research, I found no proof of the word meaning "bad stomach" or any ancient Japanese creature being called a Moruboru, as it may sound in Japanese. Mal does mean bad in Latin, and boroboro(or boro in slang) can mean damaged or rotten in Japanese. Perhaps it is a combination of Japanese and Latin for something "horribly rotten" and truly is merely a word crafted by the creative team of FFII that has built into a fantastic mythos. In short, Malboro means a giant green tentacle monster with a large mouth that spews Bad Breath, and that's about it.