Both differ genetically enough from the domesticated variants and took different paths thousands of years ago. In fact it is not sure the Przewalskis were from a domesticated group. They also show a huge lot of characteristics common in wild horses and may be a mix of the last remainders of wild living horses in Europe and domesticated ones. But there are enough other examples. In birds there are about 4000 proven examples of hybridization, half of it due to captivity, the other half occuring naturally, but the numbers are estimated to be higher as it is not always possible to identify wild living hybrids. Also we humans are hybrids, there's still Neanderthal DNA found in us.