Mg is sort of like teeny tiny Emacs.
It comes from BSD and is totally not associated with the GNU Emacs (the documentation makes that very clear).
From line 20 of the README,
This program is intended to be a small, fast, and portable editor for people who can't (or don't want to) run real Emacs for one reason or another. It is compatible with GNU because there shouldn't be any reason to learn more than one Emacs flavor.