Credit to Adam Vitale's guide for the information on which I based this guide:
Final Fantasy Tactics: The Ivalice Chronicles – Poaching Guide (RPG Site)[www.rpgsite.net]

Unlocking Poaching

• Poaching becomes available starting in Chapter 3.
  
• Equip the Poach Support ability from the Thief job.
  
• When a unit with Poach lands the final blow on a monster with the normal "Attack" command, its carcass is added to your inventory.
  
• You can obtain a carcass from both enemy and friendly monsters.

This map is from the original PSX version, but the locations are the same. (Credit to VGCartography on DeviantArt for the map[www.deviantart.com]). The locations with a Poacher's Den are circled in red:


Common vs. Rare Poach Items

• Each monster species yields one of two possible items:
  
  • Common item
    
  • Rare item (marked with a ✨ icon next to the name of the monster when obtaining its carcass)
• For the achievement, you must collect 10 different poach-only items (whether they are common or rare).
  
• The carcasses that provide a poach-only item are marked by a monster icon at the poacher's den.

Easiest Method: Breeding

• Instead of relying on random encounters, recruit monsters to breed them:
  
  • Use an Orator’s Entice (or character equipped with Speechcraft) with Beast Tongue ability equipped, or anyone with the Tame (Support) ability (requires getting the monster down to critical health).
    
  • Recruited monsters will lay eggs as you travel the world map.
    
  • Each species has three palette-swapped versions (e.g., Chocobo → Black Chocobo → Red Chocobo, Dragon → Blue Dragon → Red Dragon).
    
  • Breeding allows access to all variants of a species from a single captured monster.

Efficient Poaching Strategy

• Save your game before battle.
  
• Bring the monster(s) you wish to poach and a unit with the Poach ability.
  
• Defeat your monsters using a regular attack with the character equipped with Poach.
  
• If you receive the carcass you want, finish the battle and visit the Poacher to claim them.
  
• Otherwise, flee and retry until satisfied.

Achievement Tips

• Only 10 unique “poacher-only” items are needed.
  
• Some of these qualifying items may actually come from “common” carcasses—check for the monster symbol in the poacher’s menu to confirm.

The items that are in bold and underline in the below table are not sold by shopkeepers, and therefore count towards the "Rare Wares" achievement.


Midlight's Deep: After completing the Mullonde Cathedral battles (in Chapter 4), you return to Warjilis, triggering a cutscene that unveils the island containing Midlight’s Deep on the world map. In other words, it is a post-game dungeon.

A pig CAN be found in a normal Dorvaulder Marsh battle! It is rare, but if you missed the pig in the scripted battles, this is how you can get one.