Roy Dobbs was once the sheriff in the town of Bristol, Rhode Island. He helped Sam Winchester and Samuel Campbell hunt an arachne, but was betrayed and was turned into an arachne himself.
History
Sam, Samuel, Roy and Brenda talk.
During Sam Winchester's soulless period, he and Samuel Campbell were hunting an arachne that was taking men in their thirties, and poisoning them in order to turn them into arachne to presumably feed on. They tracked the arachne down to Bristol, Rhode Island, where they enlisted the aid of Roy Dobbs. Sam, being soulless, used Roy as live bait and let the arachne capture him so that he and Samuel could track the monster to her lair. There, Sam killed the female arachne by decapitating her. Roy, being eaten alive, is told by Samuel that he'll be taken to a hospital but is stopped by Sam, who says they're dead men walking. He suggests killing them and burning the bodies as to remove the evidence. Sam calls Roy a hero and mentions that he saved a lot of lives, and Roy, begging for his life, is shot along with the other arachne victims. Sam and Samuel burn the building with the victims inside. Unbeknownst to them, the infected men, including Roy, survived and managed to escape.
Roy Dobbs' decapitated head
A year later, Sheriff Roy Dobbs, now an arachne, lures Sam back by taking women that Sam slept with, in order to exact revenge. He has also infected the women and turned them into more arachne. Sam, who has regained his soul, initially remembers nothing of the original hunt, but it returns to him in flashes of memory. Dobbs catches Sam and Dean and reveals that the arachne wasn't feeding, but breeding, and he and the other victims escaped with their lives. He also has come to believe that Sam and his wife, Brenna, have been sleeping together and the whole reason he got rid of him was to get to her. He eventually went on to brag about how he turned women Sam slept with, and with the help Brenna, they escape, and Sam decapitates him.