M. rex

M. rex may refer to:
 * Megalopanax rex, a flowering plant species endemic to Cuba
 * Melanocharacidium rex, a species of South American darter
 * Meriones rex, the king jird, a rodent species found in Saudi Arabia and Yemen
 * Mormula rex, a species of sea snail
 * Mylomys rex, the Ethiopian mylomys, a rodent species found only in Ethiopia
 * Myodes rex, the Hokkaido red-backed vole, a rodent species found only in Japan
 * M. Rex, a comic book series that inspired Generator Rex.