Taking a look at shapeshifters. According to my encyclopedia of magical creatures, shape shifters are part of many cultures that depended on hunting for survival. Many hunters, in order to be successful will follow in the tracks of the animals they are hunting and they will also run like that animal and imitate the sounds. It is not surprising that we have made up stories depicting shapeshifters. Perhaps the first stories were about the hunters who emulated an animal so perfectly it was difficult to distinguish the hunter from the prey.
I can see the men sitting around the campfire, dancing, laughing and boasting about this ability. In Irish legend, the hunters ability to meld into the scenery around him was not only vital but was the hunter was described as being invisible. Could this be where the magical invisibility cloak was derived from? Hmm...
Then we come to shapeshifting spells. Fith-Fath as the Gaelic hunters called them were employed when they actually wanted to become the animal. This pagan spell was said to be utilized by St. Patrick himself, who invoked the spell to escape from his enemies. According to the old tales he changed himself and his followers into deer.
Only extraordinary individuals are able to shapeshift. In Irish legend the foremost of these people was a man known as Tuan Mac Carill. Tuan was known to shift into a stag, a boar, and a salmon.