On earth, muons are created when a charged pion decays. The pions are created in an upper atmosphere by cosmic radiation and have a very short decay time--a few nanoseconds. The muons created when the pion decays are also short-lived: their decay time is 2.2 microseconds. However, the muons have high energies, so the time dilation effects of special relativity make them easily detectable at the earth's surface.
As with the case of electrons there is a muon neutrino which is associated with the muon. Muon neutrinos are denoted by νμ.
Positive muons can form a particle called muonium, or μ+e–. Due to the mass difference between the muon and the electron, muonium is more similar to atomic hydrogen than positronium. Muonium has been used to produce muon-catalyzed fusion in which muons shield the positive charge of the nuclei so that the nuclei can fuse.
Reference: Serway & Faughn, College Physics, Fourth Edition (Fort Worth TX: Saunders, 1995) page 841