I can't believe I can't find this on Google... 

But if the party kills a dragon and wants to harvest its scales or teeth or whatever.. or some snake monster and they want venom; what do they roll? Are specific tools required? Does Proficiency come in to play?

There aren't really any official rules in DND 5e for harvesting parts from monsters. There is a great set of guides for it on DMs Guild! 

It's the Monster Loot series. 

I was afraid of that; wow... seems like a massive oversight on wotcs part in creating this game... I would assume an overwhelming majority of games has someone who would attempt to do this, it's such a common option in games in general; kill a monster, get its body parts, craft or sell them.

Just like substantial magical item crafting, for 5e there are no RAW rules for harvesting (there may be a rule for taking something inside certain monster descriptions but I can't think of any off the top of my head).  Some editions did have official rules (though outside the core books IIRC) which can be mined for inspiration, and plenty of third party publishers have produced systems to harvest from fantastic creatures ready to go for 5e.  

But I'd likely use nature or medicine or survival for harvesting mundane creatures, depending on the circumstances and use, and lean into arcana for more magical creature, and set the DC for the check commensurate to the value/potency of whatever it is the PC is harvesting.  "Harvesting" also may have timetables beyond "I take the x", hides would likely need curing for example.  Alchemy kits or other tools can be used as they seem useful.

I assume you mean you can't find official rules for it with a Google search.  I see several homebrew rules out there for monster bits harvesting. 

Never mind, just realized the title of your thread mentions "raw rules".  My bad.

Quote from Tibryn >>

I was afraid of that; wow... seems like a massive oversight on wotcs part in creating this game... I would assume an overwhelming majority of games has someone who would attempt to do this, it's such a common option in games in general; kill a monster, get its body parts, craft or sell them.

Eh, honestly not so much. D&D isn't really built with hard systems for those kind of trade and survival mechanics; payouts generally come in the form of currency (either explicit or treasures that exist only to be converted into currency), magic items, and experience. Actually breaking down a body and using the parts is much more a video game system than a tabletop one, in my experience. It's a lot of moving parts for a person to keep track of, particularly if that's in addition to the usual treasure payout.

I don't think it deserves to be called a "massive oversight". It's just that you don't really need to write down a rule for every little piece of minutae that a DM should be able to make up an answer for on the spot. Kinda like . . . what's the rule for how often player characters have to go to the bathroom? There isn't one. That's not an oversight. We just don't need to roll dice and consult a chart for things like that. You just killed an adult red dragon? Cool! You wanna loot the corpse? Cool! Two of you spend the next 30 minutes looting the corpse and you end up with 6 claws, 14 teeth (2 of them the big incisors), four vials of blood, and enough hide to make a suit of armor. Oh, and maybe an eyeball. Because you never now when you're gonna need a dragon's eyeball. Might come in handy. 

If a particular player has a particular purpose in mind for a particular part of the corpse, then adjust accordingly. If they're in a hurry, give them less. If they take their time, and they're proficient in some relevant skill, give them more. 

Easy peasy. It's a game. Make it up.

It's not an oversight, it's an exclusion.  D&D hasn't had rules for harvesting monster parts since 2nd Edition, after which point they decided not to assign value to such things and abstract the way magic items are created rather than say that you had to hunt down X number of monsters and harvest their spleens in order to create a magic axe.

It's worth noting that 5e follows the Legend of Zelda (ore BotW) mentality rather than Elder Scrolls - you don't get goodies as much from dead bodies so much as from chests etc.

As such, I don't think it's a massive oversight. However, it is a problem - there are monster parts quests that would be benefitted a lot by those rules. They don't happen very often in my experience, but they do occasionally. One thing to bear in mind is that if you're using official modules etc then they generally don't account for this kind of activity - if you allow players to get an income from this, you need to ensure that other sources of income are reduced to compensate. At least in the early levels - later on, they're swimming in gold anyway so it doesn't matter.

i guess you're right and my opinion was badly phrased... i guess i just assume, since dnd rules are generally very nuanced, and sometimes DO seem to account for everything that something this "common" (i feel common anyway) isn't included. 

personally, i feel like its something druids or hunters should be good at.. would be another good use for nature/survival checks that i dont see asked for very often. 

One possible reason for the lack of current rules regarding the harvesting of monster corpses is that most spellcasters in 5e use a spellcasting focus, which negates the need for many of the material components listed in the spell descriptions. Back in the olden days if you wanted to cast a spell, you needed the components - period. You couldn't cast Fireball until you had harvested some guano from a bat cave. you couldn't cast Acid Arrow until you had harvested an adder's stomach. You couldn't cast Mass Suggestion without a snake's tongue. 

So casters were constantly harvesting every piece from everything the party killed, because they needed those pieces simply to function! But the new spellcasting focus does away with the need for all that harvesting and bookkeeping. It's part of the new edition's way of streamlining the game, I guess. Personally, I like the olden days of harvesting pieces. Because heck, some monsters don't carry coins or gems on them. If you kill a basilisk, it's probably not gonna have a treasure chest filled with coins in its lair. But if you can manage to pop out one of its eyeballs, that'll be sure to fetch a sweet little profit at some shady market somewheres. 

But that's a dimension of the game that we DM's just have to manage ourselves now. 

I can see a whole campaign arc where the party is hired by one Guild or another to hunt down some monsters for specific spell components. One of the issues with this is scale. Most material components that don't have specific details (like a statue worth x thousand of gp), mention small amounts like a pinch or a handful. How much of that substance can you get from the source creature? 

One worry about asteroid mining IRL is that one good haul might break a facet of the world economy. Now convert that to your world. If ground dragon scales are sold by the pinch, and a scale yields 10 pinches, how much is one adult dragon's scales worth? THEN you need to figure out how to carry it...and process it...and distribute it.

Geeks like me think about this but I can see many DM's wanting to avoid such details.

As others have said that is not a part of the game anymore via RAW. Off the top of my head I can think of only 2 magic items that actually call for harvesting for their creation - Dragon Scale Armor (need dragon scales/hide) and cloak of Displacement ( needs/used to need displaced beast hides). Then there are things like skill with an herbalism kit (and alchemy kit) allowing you to create potions of various sorts, and the harvesting of poisons from various monsters but most of that is handled in the monster descriptions not in a rules section. Older editions had more harvesting info with things like cave fisher silk being harvested to make extra strong ropes etc. a part of the problem is any sort of official harvesting system is going to annoy some part of the audience that will then homebrew around it so why not just let us homebrew from the get go? Then you have to think about the crafting from the parts - even something like dragon scale armor - did you harvest from a juvenile or an ancient wyrm? The scales are going to be very different in size and the skin is going to be very different in thickness. Do you just want the scales? Or do you need the skin too? If you need the skin do you actually have to have the scales or could you make Dragon leather armor or dragon studded leather armor as well ( in my world you can) . Is it really scale armor if you are working with 5‘ long wyrm scales that you have to carve to fit the individual? 
All sorts of additional problems that are easier to leave to the DM’s discretion than to try and create general rules.

You could also use leatherworker’s tools for harvesting skin/scales, and cook’s utensils for meat.

Quote from AnzioFaro >>

One possible reason for the lack of current rules regarding the harvesting of monster corpses is that most spellcasters in 5e use a spellcasting focus, which negates the need for many of the material components listed in the spell descriptions. Back in the olden days if you wanted to cast a spell, you needed the components - period. You couldn't cast Fireball until you had harvested some guano from a bat cave. you couldn't cast Acid Arrow until you had harvested an adder's stomach. You couldn't cast Mass Suggestion without a snake's tongue. 

So casters were constantly harvesting every piece from everything the party killed, because they needed those pieces simply to function! But the new spellcasting focus does away with the need for all that harvesting and bookkeeping. It's part of the new edition's way of streamlining the game, I guess. Personally, I like the olden days of harvesting pieces. Because heck, some monsters don't carry coins or gems on them. If you kill a basilisk, it's probably not gonna have a treasure chest filled with coins in its lair. But if you can manage to pop out one of its eyeballs, that'll be sure to fetch a sweet little profit at some shady market somewheres. 

But that's a dimension of the game that we DM's just have to manage ourselves now. 

 Even back in 2nd Edition, I never had a GM who actually cared about that level of detail.  If a spell's material component didn't have a value listed in gold (like the pearl for Identify), they'd typically ignore it as long as the wizard had their spell component pouch on them.

It's not actually true that there are no RAW rules for harvesting. They're just very limited. Specifically, there are rules for harvesting poison.