There is an old man lurking in the Forest, Jungle, and Swamp zones and will trade you specific items for ones you have on-hand. You can only trade with him up to 3 times per visit.

Forest and Hills - Area 7
Insect Husk -> Special Mushroom
Cactus Flower -> Special Mushroom
Felvine -> Special Mushroom
Fire Heb -> Special Mushroom
Iron Ore -> Small Monster Bone
Machalite Ore -> Medium Monster Bone
Dragonite Ore -> Large Monster Bone
Crimson Horn -> Wyvern Marrow
Majestic Horn -> Rathalos Wing
Lightcrystal -> Rathian Spike
Kut-Ku Shell -> Plesioth Scale
Kut-Ku Ear -> Plesioth Fin
Cephalos Fin -> Rathalos Webbing
Blos Fang -> Large Monster Bone
Piscine Jaw -> Medium Monster Bone
Diablos Spine -> Gravios Carapace
Gravios Carapace -> Diablos Spine
Gravios Head -> Vespoid Abdomen
Gravios Wing -> Vespoid Abdomen
Monoblos Spine -> Wyvern Marrow
Rathian Plate -> Lao-Shan’s Scale
Rathalos Plate -> Lao-Shan’s Shell
Fatalis’ Eye -> Kirin Hide
Firestone -> Rathalos Wing
Pale Lips -> Pale Bone
Mystery Bone -> Stunshroom
Garbage -> Fire Herb
Pawprint Stamp -> Firefly

Jungle - Area 5
Insect Husk -> Special Mushroom
Cactus Flower -> Special Mushroom
Sushfish -> Special Mushroom
Fire Herb -> Special Mushroom
Rathalos Claw -> Demondrug
Rathian Claw -> Armorskin
Rathalos Scale -> Catalyst
Rathian Scale -> Catalyst
Rathalos Shell -> Dragon Toadstool
Rathian Shell -> Dragon Toadstool
Piscine Fang -> Power Extract
Blos Fang -> Mega Juice
Blos Jaw -> Pale Extract
Mosswine Head -> Max Potion
Mosswine Hide -> Max Potion
Bullfango Head -> Max Potion
Flame Sac -> Demondrug
Electro Sac -> Immunizer
Poison Sac -> Lifecrystals
Stun Sac -> Catalyst
Sleep Sac -> Armorskin
Wyvern Marrow -> Ancient Potion
Small Lobstershell -> Mega Armorskin
Large Lobstershell -> Mega Demondrug
Mystery Bone -> Toadstool
Garbage -> Screamer
Cephalos Fin -> Twisted Horn
Plesioth Fin -> Twisted Horn
Pawprint Stamp -> Mega Bugnet

Swamp - Area 1
Insect Husk -> Special Mushroom
Ivy -> Special Mushroom
Blue Mushroom -> Special Mushroom
Honey -> Special Mushroom
Small Monster Bone -> Iron Ore
Medium Monster Bone -> Machalite Ore
Basarios Shell -> Machalite Ore
Gravios Carapace -> Dragonite Ore
Rathalos Wing -> Lightcrystal
Rathalos Claw -> Machalite Ore
Rathian Claw -> Machalite Ore
Pale Bone -> Union Ore
Gravios Head -> Firestone
Gravios Wing -> Large Lobstershell
Basaios Wing -> Small Lobstershell
Lao-Shan’s Spine -> Firestone
Kirin Mane -> Lightcrystal
Rathalos Tail -> Machalite Ore
Wyvern Marrow -> Lightcrystal
Hornet Bladefin -> Union Ore
Vespoid Bladefin -> Union Ore
Kut-Ku Ear -> Machalite Ore
Giant Beak -> Lightcrystal
Rathian Spike -> Machalite Ore
Mystery Bone -> Sleep Herb
Garbage -> Sap Plant
Pawprint Stamp -> Mega Pickaxe
Vespoid Abdomen -> Novacrystal

Credit: S HIRYU from GameFAQs

RIO Reus = RATHALOS

Someone finally posted a short FAQ about this on GameFAQs.
link removed

Not sure how accurate it is, but it looks pretty decent. Perhaps we'll "borrow" those lists and just make a sticky post here for quick reference.


EDIT: Added the lists up top.

That list saved me. Thanks for the heads up.

Printing list right now,thanks Mute.

I really should have looked at this earlier >_>. Ah well, now I know.