This is something we probably wont get an answer to. The foretold was simply a thing for Gus to study, a reason to get the Doctor and all the other great minds onboard the doomed train. But that doesn't stop us from asking some questions about the creature and where it came from. What war did the soldier fight in? Does it have an ancient Egyptian connection as the episode - and The Big Bang - suggested? If there is more to Gus then perhaps there is more to the foretold and the answers may lie in the war he or she fought in. If the rumours of rebel timelords are true, could this in fact be a soldier from the Time War? It would certainly explain how the Doctor was able to quickly solve the mystery of the mummy.