Who is The Glashtyn from The Girl Who Circumnavigated Fairyland in a Ship of Her Own Making and what is their importance?

The Glashtyn is a race of water dwellers that monitors and controls the passage of ships across the river surrounding Fairyland's capital, Pandemonium. As their unpredictably-taken fee for allowing ships to pass, the Glashtyn (who have the bodies of humans but the heads of horses) take a child to be their servant. When the boat September is on is raided by the Glashtyn, she sacrifices her shadow so that the child chosen by them to be their servant can remain free. In short, the Glashtyn are catalytic characters, their actions creating a situation in which the narrative can, through the actions of September, explore the thematic issues of self-sacrifice and choice.