Mg is a lightweight public-domain ErsatzEmacs, dating back to 1986. Mg was originally known as MicroGNUEmacs, as it strove to adhere more closely to the default behavior of GnuEmacs than other contemporary MicroEmacsen, but was renamed at the request of RichardStallman.
Modern versions of mg are maintained as a component of the OpenBSD? source tree; the editor is included as standard in OpenBSD? “because there shouldn’t be any reason to learn more editor types than emacs or vi.”
A portable version is available for download at the maintainer's website.