Feral Pigsy Fur can be obtained from Feral Pigsies of course - either by killing them or harvesting from them after taming them to become your Pets. When taking a Feral Pigsy Fur for the first time, the crafting-recipe for Poison Resistance Potions will automatically be unlocked.
Feral Pigsies will spawn on Detritus in Jungle-biomes, but can also be found in Swamplands occasionally, not far from the Jungle; usually only during ingame daytime; they do not spawn anew at night. They are aggressive creatures that will attack your player-character when spotting him/her, and can inflict poison damage.
Feral Pigsy Fur is needed as an ingredient for a number of crafting-recipes, like for Iron Swords, Iron armor, Armor-Piercing Bombs, Poison Bombs, Bungalow Fur Walls or Bungalow Accent Walls (as of R26 in December 2015). They are not needed to craft Poison Resistance Potions though.