An antielectron neutrino is an antileptonic elementary particle - the antiparticle of an electron neutrino.

An antimuon neutrino is an antileptonic elementary particle which is the antiparticle of a muon neutrino.

There are two possible answers. Under the point-particle physics of today, a neutrino is an elementary particle, and as such, it is not composed of anything but its own material. The other answer exists under string theory. In string theory, elementary particles are all made up of the same material, one dimensional strings. These strings vibrate at different rates to create the different elementary particles. Therefore a neutrino would be a string vibrating at a specific rate unique to neutrinos. If it vibrated at any other rate, it would not be a neutrino.

Neutrinos are a product of beta decay. Some number of protons and some number of neutrons are in the nucleus of an atom. When a neutron decays, it turns into a proton, an electron and a neutrino. The protons stays in the nucleus, while the electron and the neutrino escape.

Muon neutrino was created in 1962.