Very little is known about ichrites other than the fact that they are a type of insect. They are described to feed off unicorn blood, which is presumably why Timkin knows about them (the Heks family specializes in unicorn care). It is likely that they are the Elvin world's equivalent of ticks or fleas.
“Argh—I swear I've picked up ichrites in this infernal fur.”
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 unico.