I like to think Kaepora Gaebora was Rauru, yet he can't be. Rauru, he's a sage, he's known that the hero would come, and known the role he, and the sages would have to play. In the Spirit Temple the Owl says to Link he thought a hero travelling through time wasn't actually possible, but he was wrong. Could a sage be so wrong in something they should know????

Bringing a second theory to the ranks, some people believe, and I find this theory more feasible, that the Owl is the Deku Tree's re-incarnation so to speak. Not necessarily a re-incarnation, yet throughout Zelda people do survive in some form after their death. King Hyrule as KORL, OoT Link, as the dude who teached Link moves in TP, and why not the deku tree as the owl.

The owl appears right after the Deku Tree dies, and speaks with wisdom, as the deku tree did, though somewhat different. The Owl already knew Links name too!!!

I see the owl as Links guardian who watches over him, even into Termina. I don't think Rauru is going to do that, I think the deku tree would because he fulfills his duty.. Links mother intrusted him to the deku Tree, and I think he would fulfill that duty, even in death, through a different form. He isn't in later games, such as WW because he was only the OoT Links gaurdian.

On an other theory if Hyrule has guardian spirits or angels on each person, Kaepora Gaebora is Links!!!