Having recently decided my Bazaar trader needed a new look, I attempted to finish this quest after a lapse of about twenty years. For the sake of the Everquest community (do these quests exist on the TLP servers?) I'll share here what I learned.
The spawn rate on the giant sylvan bats is terrible and the drop rate is even worse. After running around with my ranger, tracking and killing every bat and arborean in GFay, I decided to hang out instead on orc hill, because many of the arboreans and a couple of bats had roamed around there and the Crushbone entrance. Standing in the middle of orc hill at the top of that ridge, I faced west and killed every mob that moved, completely ignoring the static spawns and the enslaved miners (YoDP quest spawns). By that time I had the arborean drops I needed, so I stopped killing those, ultimately ending up with eleven "a mature arborean", a dozen "an arborean saplling" and a few "an arborean sprout" on track, most of which pathed right past me on the hill. They do not appear to be placeholders for the giant bats, as the giant bats continued to spawn without regard to how many arboreans were up. The ground was littered with mob corpses and I was falling asleep, but I had one Giant Sylvan Bat Fur and wasn't about to give up!
When I logged in the next morning, I was the only person in the zone and there were two "a giant sylvan bat" on track, both near orc hill. I killed the first, but decided to simply follow the second to see where it pathed. It consistently moved between the east and west zone walls, pausing along the way to change trajectory slightly but never straying far from that east and west pathing. After several trips I killed it, too, and of course it dropped nothing. Back on orc hill, I started again killing any mob that moved, including the arboreans this time. Eventually a giant sylvan bat spawned right behind me and slightly on the downhill side toward Crushbone and dropped the final bat fur I needed.
Important note: If you see "an elven skeleton" be sure to kill it right away. They attack roaming orcs of all kinds and giant sylvan bats, and I saw one kill a bat I'd been tracking before I could get to it to intervene. As though the spawn rate on those giant bats weren't bad enough already, right?
Good hunting!