Ichrites are a type of insect, likely the Elvin world's equivalent of ticks or fleas. They are described to feed off unicorn blood, which is presumably why Timkin knows about them, as the Heks family specializes in unicorn care.
Shannon Messenger's Description in Unlocked[]
A type of tiny, troublesome insect similar in size to a flea, but with silver bodies perfect for blending into unicorns.