Fuego is only used once in HPMOR, by Fred Weasley (or was it George?) on the Mountain Troll that had entered Hogwarts. "'Fire and acid!' Harry shouted. 'Use fire or acid!' 'Fuego!'/'Incendio!'" (Ch. 89) Presumably it has some sort of fiery effect but Harry wasn't paying close enough attention to know what it does. It's at least simple enough for a third-year to learn. Derived from Spanish word for fire, which in turn was derived from latin Focus which can mean fireplace or hearth.