I've just been checking up and it seems that Etin is the same as the Norse Jotun. The Jotnar were one of the races of giants.
The Red Etin story seems a bit like some sort of initiation ceremony. The hero is given advice from a 'fairy' and several men looking after a variety of animals.
His task is to enter the castle, solve the riddle, win and get the girl.
Perhaps it's all part of 'Aberfeldy's' hunting society and culture?
Many years ago I was at Loch Tay on a gaelic course. We were reading from a collection of local folk tales, most of which concerned the uruisgean or 'brownies' in English. They were supposed to be water spirits who lived out on the loch.
Apparently the people living on the shore were plagued with them. One particular young uruisg used to jump down through the smoke hole in the roof and steal the bannock being baked by an old woman, before she could take it off the baking girdle. One day she places a flat stone in the shape of a bannock on the girdle. Once it is heated down jumps the uruisg who grabs the red hot stone and doesn't realise what has happened until he's half way out of the door. His mother comes round to have a go at the old woman. I forget exactly what happened but the woman won.
At the time we were there there was a very severe drought on and the waters of the loch lowered substantially. To such an extent that old posts of a crannog were exposed. Presumably where the uruisgean used to live.
Archaeologists have been at work in the area and a reconstruction of one of these crannogs has been built.http://www.crannog.co.uk/