Where can I find Fire Tarantula? (Bionis' Leg Collectibles)

I'm not sure about Believer's Paradise, but I've found them in various places around the Bionis' Leg, and not limited to night time.
There may be some places with a slightly higher chance than other places, and some places may only have a chance during the night, but it is a very rare item, so it'll take quite a while to find it.
I think I got most of mine while just wandering around Gaur Plain... there's quite a few places around there where collectables appear, and checking them all will probably increase your chance of finding it quite a bit, but it's still not guaranteed, so you may need to spend quite a while just wandering around until you find it.