I was working on a crossword puzzle with the clue bearer of gold, frankincense, or myrrh. I immediately thought that the answer is obviously either "magi" or "wise man," but then realized that the puzzle called for a 5 letter word so neither fit. Eventually I figured out that magi is a plural Latin word, so its singular form would be "magus" which was the correct answer.
Is it normal to use the word "magus" in informal, or even formal, English? I've never seen it before. I have however often seen magi used as a singular noun.