Horn antennas that support balanced hybrid-modes are desirable in satellite communication applications due to their polarization-independent patterns and low cross-polarization over a wide frequency bandwidth . Besides corrugated horns which utilize an electromagnetically soft lining, dielectric loaded horns can provide an alternative realization of hybrid-mode horns with either soft or hard wall boundary conditions -. Recently, homogeneous metamaterial liners have been used to realize hybrid-mode horn designs . With the availability of metamaterials that exhibit a dielectric constant of less than unity over a reasonably wide bandwidth, use as a liner allows the removal of the standard dielectric core. This metamaterial liner can improve the bandwidth and significantly reduce the weight of the antenna compared to its dielectric loaded counterpart, all while maintaining the performance of a hybrid-mode horn. In this paper, we show a broadband metasurface satisfying the balanced hybrid condition and its use as a liner to implement hybrid-mode horns. The design approach combines a nature-inspired optimization technique and an efficient full-wave electromagnetic solver, making it possible to realize metamaterials or metasurfaces with customized dispersive properties. With regard to surface impedances, the metasurface is found to be equivalent to a homogeneous metamaterial slab backed by a PEC ground plane. Therefore, we will consider the results of a numerical study of a conical horn antenna with a homogeneous metamaterial liner. The far-field radiation patterns and aperture field distribution confirm hybrid-mode operation over a wide bandwidth, validating the metasurface design methodology.