Riding out of the forest, after meeting the Great Dragon, Merlin's horse has a white flash on his forehead, and a dark red bridle and breast-strap. In the next scene, looking over the Isle of the Blessed, the flash has gone, and the tack is dark brown.
The tack on Lancelot's horse changes between scenes. When he talks in the forest to Merlin, his horse has a light hunting breast-strap/collar. In the next scene, overlooking the Isle of the Blessed, the horse is instead wearing a heavy medieval-style breast-girth.