Bestiary entry from the Clan Primer:
This creature always drops a trophy when it is defeated. This trophy is used in the Hunt Club side quest which involves trading in the trophies for items, armor and accessories.
|Common - (55%)||Knot of Rust|
|Uncommon - (10%)||(Nothing)|
|Rare - (3%)||(Nothing)|
* The Killbug can be found in the center of the Switchback, but it will only appear when the game clock’s minute indicator is between 10 and 39. Just leave and re-enter the area during that time to have the Killbug appear. You also need to have initiated the Hunt Club side quest.