Ok, let's look at a few facts regarding this theory. At the time of Shadow Rising approximately a year had passed. Currently in the series covers about a two year time span. Birgitt says that Gaidal Cain has been spun out and is now a baby or a little boy. So we can assume that someone can be alive in the real world _and_ be summoned to fight by the Horn. Which is some kind of paradox, but who cares? But she says he is most likely an infant or a little boy, which rules out Mat and Uno. The most likely choice left is Olver but he still had to find a way to get older. Unless some transformation occurs when the Horn is blown.