Short for Millions of Unusually Small Creatures Lurking Everywhere, was a 1980s US toy line based on the Japanese toy line, Kinkeshi which itself was based on the animanga, Kinnikuman.
The line consisted of tiny pink (though more colors were introduced later on) rubber figures, who were wrestlers from outer space. The characters were divided into two teams: The Thug Busters led by Muscleman and the Cosmic Crushers led by Terri-Bull. 236 unique figures were released in the line.
M.U.S.C.L.E proved not to be as popular stateside as Kinnikuman was in Japan. The figures remain popular through websites devoted to them, such as Nathan's Muscle Blog and Little Rubber Guys. Another notable site was I-Mockery's M.U.S.C.L.E Revisited, which was devoted to giving names and backstories (though they were slightly parodic in nature) to all 236 figures.