Ever heard of the Holyrood church in Scotland. There is a story about King David I of Scotland on a hunting party that saw a white stag that had a cross in its horns. The king was so awe struck, that he had a church built there.
welp, Stäg probably is the try to let the Word Stag sound more german. There is no correct german word for that animal anyway, as the whole race of Deer are translated as Hirsch, no special words for either sexes, although the female deer is usually called Hirschkuh (Deer Cow)