The capaill uisce are magic. Stiefvater rarely says this outright, preferring instead to suggest the lore to us. The human characters are front and center while the horses mill around in the background.
However, just because it's not beat into our brains every page how magical they are doesn't mean we forget about it. Sean often does two things to soothe the savage magical beasts: He strokes them with iron, and he ties knots in their manes. We're not sure why the knots are calming. They like having their hair braided? But iron is traditionally a metal effective against magical creatures, from faeries to witches.
The wranglers tie iron bells around the capaill, not so they can hear them coming, but because the metal weighs them down and weakens them a bit. Sean even goes as far as hitching a pair of chain-mail panties onto one of his mares. This blend of classic lore with Stiefvater's new creations adds to the magical feel of the novel.