If you are planning to craft and upgrade the Griffin armor set, then you’ll stumble upon needing monster feathers for it is required to upgrade the Enhanced Griffin Armor.
Monster feathers is also a primary ingredient for crafting crowssbow bolts.
To obtain monster feathers, you’ll need Harpy feathers. Dismantling harpy feathers on a blacksmith will then give you monster feathers.
Don’t buy “feathers” from the black smith. It dismantles into ashes, not monster feathers.
Where to farm harpies:
Harpies are located on the Harpy Feeding Ground waypoint.
Harpies don’t always drop harpy feathers though so you may need to kill a couple of them.
If you get harpy eggs, they also give monster feathers when you dismantle them.