Mushrooms are level 10 creatures that are found on an island - Mushroom Island - Reachable through the Teleporter found nearby the Slime spawn (Cost 1 Magnetic Charge, or by traveling below the docks located beneath Forest fiends in southern brightleaf.
Drops
Besides many non-unique item drops, the Mushrooms drop the Nature Amulet.
Resources
Equipment
Tertiary Drop
This can be obtained as an extra item roll rarely from all monsters. They will only drop if the monster is near your level (bosses not included)