Mechanix is Megadeth's version of the song "The Mechanix".
"The Mechanix" appears on several pre-Kill 'Em All Metallica demos. Metallica later reworked this song into the song "The Four Horsemen", which appeared on Kill 'Em All. It has different lyrics than the original song.
Megadeth then recorded the original song as "Mechanix" (dropping the word "the" from the title), but at a faster pace and a few other changes. It is closer to the original than "The Four Horsemen" is.