Bighorn sheep (Ovis canadensis) is known for its giant spiral horns that can sustain impact loading at a speed up to 5.5 m/s during ramming without causing severe damage or head concussion. The bighorn sheep horn is composed of a keratin-based biological material with a tubule-lamella structure. This special structure produces the anisotropic hardening characteristics of the horn material under impact loading. Investigating the mechanisms of energy dissipation of bighorn sheep horn can inspire the design and development of artificial materials with high capacity of energy dissipation and/or impact mitigation.In this study, a transversely isotropic constitutive model with anisotropic hardening and strain-rate effects was developed for predicting the mechanical responses of the horn under impact loading. The characterization of material properties was conducted using test data from uniaxial compression tests of the horns under both quasi-static and dynamic loadings. The constitutive model was later implemented into the commercial finite element code, LS-Dyna, as user-defined material subroutine and was successfully validated against test results. Finite element simulations were conducted on the dynamic impact against the bighorn sheep horn, and the user-defined constitutive model was used to study the mechanical responses of the horn material subjected to large impact loads without causing severe damage. The mechanism of energy dissipation was also investigated from energy absorption and conversion, stress distributions, and propagation of displacement waves.