Preservation of epididymal spermatozoa is important to save genetic material of endangered species and breeds, or in case of unexpected injury, which will end the breeding career of valuable sires. Seminal plasma (SP) influences sperm quality in a dose-dependent manner and its addition to preserved semen immediately before insemination may be beneficial for sperm fertility. Increased plasma membrane stability of epididymal spermatozoa reduces freezing injury of cells, and the addition of SP after freezing and thawing might have activating and protecting effects on spermatozoa within the female genital tract. In this study, epididymal spermatozoa were harvested by retrograde flush of the epididymal cauda immediately after routine castration and frozen. Seminal plasma was collected from other six stallions. Homologous SP (SP from the same species, but from a different animal) was added to frozen-thawed epididymal spermatozoa at concentrations of 0, 5, 20, 50 and 80% SP. Addition of SP increased sperm motility and influenced kinematic values in a dose-dependent manner (p < 0.05). Motility improved at concentrations of 20 and 50% SP, but did not further increase at 80% SP. There was no difference in sperm motility among SP from six different donor stallions regardless of the concentrations of SP (p > 0.05). Total and progressive motility of ten frozen-thawed epididymal spermatozoa samples collected from different stallions after dilution with extender and 5, 20, 50 or 80% SP differed significantly (p < 0.05). In conclusion, addition of homologous SP to frozen-thawed stallion epididymal spermatozoa immediately improved motility in a dose-dependent manner regardless of semen quality of SP donor stallions. This might positively influence fertility when SP is added before insemination. Moreover, there seems to be a threshold level of SP concentration for optimal improvement of sperm motility.