An unnamed demon possessed Peter Kent in 1958 and became a subject to Father Max Thompson's experiments.
Background
In July 1958, this demon possessed Peter Kent and ate his children, something he enjoyed greatly. The demon was eventually captured and taken by Father Max Thompson to be used in his experiments in curing a demon as his 19th test subject on August 3, 1958. For the ritual, Father Thompson would inject the demon with doses of his own purified blood each hour and then ask the demon what it felt like when he ate Peter Kent's children. After eight injections, the demon pleaded with Father Thompson to stop, losing his black eyes and demonic mannerisms and sounding more human. Recognizing that the demon was prepared, Father Thompson recited a modified exorcism, cut his hand and slapped it over the demon's mouth, causing him to emit light similar to a smiting. When it was over, the demon had been returned to being human and was horrified by what he had done. Father Thompson reassured him that while he had been a monster, he was now a man once more.
Hell eventually got wind of Father Thompson curing this demon and sent Abaddon to stop any further experiments. Abaddon killed Father Thompson but it is unknown what, if anything she did to the former demon.
2013
While looking for a way to cure a demon for the Third Trial, the Winchesters located the recording of Father Thompson's first attempt at his ritual when he failed to save an old woman. Father Simon gave the Winchesters Father Thompson's records and they listened to his recording of curing this demon. In doing so, the Winchesters were able to discover how to successfully cure a demon and proceed with the Trials.