Epitaxial films including bulklike cubic and wurtzite polymorphs of MnSb have been grown by molecular beam epitaxy on GaAs via careful control of the Sb/Mn flux ratio. Nonzero-temperature density functional theory was used to predict ab initio the half-metallicity of the cubic polymorph and compare its spin polarization as a function of reduced magnetization with that of the well known half-metal NiMnSb. In both cases, half-metallicity is lost at a threshold magnetization reduction, corresponding to a temperature . This threshold is far higher for cubic MnSb compared to NiMnSb and corresponds to K, making epitaxial cubic MnSb a promising candidate for efficient room temperature spin injection into semiconductors.