VERY common. it takes a well trained eye to spot a specific species in their mixed tanks. most red zebras are pure, ebjohanni, and bumblebee's, but these guys are also the most common hybrids (among others). we may not have a definitive answer to what's in his blood, but we can tell you that he's a hybrid. personally, i like hybrids just for the fun of the mystery! if i could, i'd have a tank of all my adult hybrids so that i wouldnt have to worry about them with my pure species (some are easy to mix up, like my 'icefin zebras' have yellowy-orange bodies and bright metallic ice-blue fins. an untrained eye would think it's just a mutated zebra! its actually an F1 hybrid between a male kennyi and female red zebra)
good luck with your fish!