The carriers of the weak force (W and Z) were not hypothesized by Yukawa
but came in much later. Yukawa was the first to propose the existence of
a "meson" something of middle mass (between the electron and the proton)
that would carry the strong force. When the muon was discovered in
cosmis rays it was at first thought this was Yukawa's meson, except that
it turned out not to interact strongly. Only later when the pion was
discovered was Yukawa's hypothesis supported.
The standard model nowadays says that the fundamental carrier of the
strong force is not the pion but the gluon, which mediates the force
between quarks, in the theory called QCD (Quantum Chromodynamics).
J. Epstein