Are Mesons Elementary Particles?

Ordinary mesons are made up of a valence quark and a valence antiquark. Because mesons have spin of 0 or 1 and are not themselves elementary particles, they are "composite" bosons.

What particles are mesons?

Meson, any member of a family of subatomic particles composed of a quark and an antiquark. Mesons are sensitive to the strong force, the fundamental interaction that binds the components of the nucleus by governing the behaviour of their constituent quarks.

Which is not elementary particle?

X-rays are not particles, they are electromagnetic waves having higher energy than UV and less energy than gamma rays. So, they are not fundamental particles.