Joseph "Old Man" Finnegan was an old man who died some time prior to The Boogeyman.
Not much is revealed about Finnegan's early days, but the many hunting trophies and rifles in his home suggests that he was an avid hunter. One day, his wife left him for another man, turning him into a reclusive hermit. He would spend several hours a day for the following decades just sitting by his window, hoping for her to return. Because of his lonely, isolated nature, he became the subject of many local horror stories portraying him as an old boogeyman who would abduct children, hold them captive in his basement, kill them, and then skin their bodies and eat them. Sometime prior to "The Boogeyman", he died of a heart attack, his death remaining unnoticed by everyone except young serial killer Jeffrey Charles, who buried his body under a heap of leaves on the premises and used his house as a hideout, bringing in packaged meals delivered by the local church community and eating them in order to keep up the appearance that Finnegan was still alive.
When the BAU is called in to investigate the murders, they learn of the local horror stories about Finnegan and consider him a potential suspect. Going to his house, they find his dead body outside, clearing him of any suspicions.