I created a text table for Might and Magic and loaded it onto the ROM in a hex editor, and sure enough, at the end of the list of enemies, is SHELTEM.

This not only confirms that unused "boss" is Sheltem, but he might even be coded in the game. Oh, if only my hacking skills could go so far as to activate him somehow. I do not believe that the other three enemies are coded, though. There are no unused monster names that could correlate to them.

BTW, that site I linked to above claims he could not find the Water Rat, Brontasaurus, and Pirate Captain enemies, but they are in the list, and while I'm not sure about the others, I could swear I've seen the Water Rat myself (think it had the same sprite as the Savage Shrew/Battle Rat.) If Pirate Captain and Brontasaurus exist, they must share sprites with someone else because those unused enemies wouldn't fit.