Humans cannot get impregnated by a horse, or any other species for that matter, due to genetic and physiological differences between species. The process of reproduction and the formation of a new individual (offspring) involve the fusion of male and female gametes (sperm and egg) during sexual reproduction. For successful reproduction, the genetic material of the male and female must be compatible.
In mammals, including humans and horses, the genetic differences are significant enough that their gametes cannot fuse and produce a viable embryo. Each species has its unique set of chromosomes and genetic makeup, and the reproductive barriers prevent the mixing of genetic material between species.
The concept of interspecies reproduction (hybridization) exists in some cases within the same family or closely related species. For example, certain species of horses can interbreed to produce hybrids. However, humans and horses are not closely related enough biologically for successful reproduction to occur between them.
Aside from genetic incompatibility, there are numerous physiological differences between humans and horses that prevent the possibility of pregnancy. The structures of the reproductive systems in humans and horses are specifically adapted to their own species, and they are not designed to function together in this manner.
Overall, the idea of interspecies reproduction between humans and horses is purely a fictional or mythological concept and is not supported by biological science. Reproduction only occurs successfully within a species due to the compatibility of their genetic and physiological makeup.