The freiwillige Jager detachments were small (often around 64 men) groups of mid-class people and often students. They brought their own uniforms and weapons, and served for as long as the war went own(Not sure on that list bit though).
Lutzows freikorps had a rifle detachment, but they were part of said unit, so they aren't exactly the same.
The concept of 'Freiwilligen' isn't just Prussian btw. Some Dutch cavalry had them, as well as the 1st Nassau infantry.