  The magnetism of the rock-salt $fcc$ rare-earth monopnictide HoBi, a
candidate topological material with extreme magnetoresistance, is investigated.
From the Ho$^{3+}$ non-Kramers $J$=8 spin-orbital multiplet, the cubic crystal
electric field yields six nearly degenerate low-energy levels. These constitute
an anisotropic magnetic moment with a Jahn-Teller-like coupling to the lattice.
In the cubic phase for $T>T_N~=~5.72(1)~K$, the paramagnetic neutron scattering
is centered at $\mathbf{k}=(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ and was fit to
dominant antiferromagnetic interactions between Ho spins separated by $\{100\}$
and ferromagnetic interactions between spins displaced by
$\{\frac{1}{2}\frac{1}{2}0\}$. For $T<T_N$, a type-II AFM long-range order with
$\mathbf{k}=(\frac{1}{2}\frac{1}{2}\frac{1}{2})$ develops along with a
tetragonal lattice distortion. While neutron diffraction from a multi-domain
sample cannot unambiguously determine the spin orientation within a domain, the
bulk magnetization, structural distortion, and our measurements of the magnetic
excitations all show the easy axis coincides with the tetragonal axis. The
weakly dispersive excitons for $T<T_N$ can be accounted for by a spin
Hamiltonian that includes the crystal electric field and exchange interactions
within the Random Phase Approximation.
