This wouldn't be based on any book at all this would be respect the old days of the Celtic states such as Scotland and Ireland. Tapestries of unicorns plus Scotland's flag, and then let the magical adventure be the triplets go on their own the see the witch and get the food that'll turn them into bear cubs, so they can hurry to help Merida and Macintosh while they're trying to save a horse or pony from this magical horse cousin the Kelpie it could be Angus or a cousin from the wild.
The film could start like showing a calm kelpie photo and saying There was once a magical horse between the coasts of scotland and Ireland called the kelpie. Next let the sequel show unicorn and kelpie stuff like the first film showed several things related to Mor'du.
This idea respects Merida's culture even more than the beat story does. the kelpie would be introduced from its original area of Europe showing it in a film like this film of Brave. The challenge would be similar the first film though. In the imaginary world the Kelpie like to eat one gender more than another, and it's one of the active scary water cousins in fantasy but the unicorn's indeed more from European stuff than any other continent, that's what gives me the unicorn tapestry idea for this sequel of Brave. So if shown in more than just tapestries there could even be a horse coming to the rescue as brave as a unicorn would after noticing the danger Merida's really in with Young Macintosh and her triplet brothers. The regularly fast horse would have no trouble outrunning the kelpie and escaping before they were actually chased, it's scenes of seeing the kelpie near the water that would be the scary scenes like the scenes of Merida in caves showed scarier bear scenes of scenes of her mom as a grizzly at their castle. This would be the type of tale to show anyone can be a hero.