Donald Deagle was the husband of Ruby Deagle and uncle of the spoiled Douglas Deagle. He died somewhere prior to the events shown in Gremlins. What little background is known of him is from a brief radio news report of Mrs. Deagle's death at the hands of the gremlins, noting that Donald Deagle was charged with selling worthless stock and ultimately convicted of fraud. Sometimes later he died and earned the reputation as a disgraced stock swindler. It was unknown if Mrs. Deagle had also participated in the fraud, although it seems likely that Mr. Deagle hid assets in his wife's name. As such, the Treasury and FBI were unable to seize Deagle Estates and like holdings in the subsequent asset forfeiture from his conviction.
Although he wasn't seen personally in the film, a large framed photo of him could be seen hanging in the Deagle Real Estate where Ruby was living up until her death in the gremlin incident that plagued the town.