Kelpie hair

"Early in my career, as I watched my wandmaker father wrestling with substandard wand core materials such as kelpie hair, I conceived the ambition to discover the finest cores and to work only with those when my time came to take over the family business."

- Garrick Ollivander

The hair from a kelpie can be used to comprise the core of a wand, but is known to be a terrible core.

Wandlore
Kelpie hair core is noted to be an inferior substance.

Known uses

 * In his time as owner of Ollivanders Wand Shop, Gervaise Ollivander used to make wands of Kelpie hair, which inspired his son Garrick Ollivander to be a wandmaker himself.