  Flavor-exotic tetraquark mesons have recently been observed in the
heavy-quark pair sectors of QCD, including two isospin multiplets in the b
b-bar sector, Z_b(10610) and Z_b(10650), and one isospin multiplet in the c
c-bar sector, Z_c(3900). We identify Z_b and Z_c as tetraquark mesons that are
analogs of quarkonium hybrids with the gluon field replaced by an isospin-1
excitation of the light-quark fields. Given the identification of Y(4260) and
Z_c(3900) as a ground-state charmonium hybrid and tetraquark, respectively,
lattice QCD calculations of the charmonium spectrum can be used to estimate the
masses of the lowest four spin-symmetry multiplets of charmonium hybrids and
tetraquarks. The Z_b(10610) and Z_b(10650) can be assigned to excited-state
multiplets of bottomonium tetraquarks, resulting in estimates of the masses of
the ground-state multiplets of bottomonium hybrids and tetraquarks.
