The pig farmers are three bootleggers whom Rocky encounters in "Rumrunner". Based on dialog it is clear that they have had prior run-ins. In "Hydrophoby" one of the farmers is identified as Avril. In "Sunshine" another one of the farmers is addressed as Emery. Avril appears to nominally be their leader, and based on their interactions they may be related.
After confronting Rocky in "Rumrunner", the farmers abduct him in "Hydrophoby" and place him in the back of their truck with "Frothy", a rabid pig that Avril was forced to shoot. In "Crack-Track" the farmers intend to kill Rocky by pinning him to a set of railroad tracks with nails through his tie, but he escapes in "Face-plant".
In "Scathefire" through "Schadenfreude", Rocky and Freckle visit and ultimately destroy the farm where the pig farmers live and work. The farmers discuss the attack and how to respond in "Lambaste", with Avril noting that "some people" owe them a favor.
It is revealed in "Muckamuck" that, shortly prior to the timeframe of the comic, the pig farmers received a large sack from Mordecai, Nico and Serafine and presumably fed the contents to the farmers' pigs. In the present, Avril tells Emery to get Benjy Jessup and Jessup's truck, presumably as part of Avril's plan to deal with Rocky.