"Mad Dog" Morgendorffer
"Mad Dog" Morgendorffer was the deceased father of Jake Morgendorffer. We do not know the real name of him, only his grotesque nickname (a reference to the film Mad Dog Morgan by the writers). While it wasn't stated outright on the show, Glenn Eichler has said he was supposed to have been in the U.S. Army as part of the draft. He died in his early-to-mid 40s, after Jake and Helen married. "The Daria Hunter" mentions he wore contacts.
“Mad Dog” does not appear in the Daria show, but his legacy does in the form of Jake, Daria and Quinn’s neurotic father. Mad Dog's presence is felt in many episodes in the series, starting in "The Teachings of Don Jake," as Jake rants at the way his father treated him. We know he took Jake camping but wouldn't allow them to use tents, and got wasted on beer during the camp trips ("The Teachings of Don Jake"); that his father would basically ignore Jake when he got hurt and tell him to man up ("Monster"); and that he would try to force Jake to be more masculine and self-sufficient, not relying on or expecting anything from others, a trait Mad Dog viewed as a weakness ("Of Human Bonding"). The end result was that Jake was still wetting the bed at age 15 ("The Teachings of Don Jake").
Despite all of this, Mad Dog recorded Jake's childhood on numerous home movies (and thus recorded at least one instance of him berating his son). ("Monster")
During Jake's childhood, Mad Dog sent him to Buxton Ridge Military Academy - the day after Jake stepped on one of his contact lenses by accident, according to "Daria Hunter" - and rarely if ever visited him. At least once Mad Dog left Jake there during Christmas to make him 'self-sufficient,' and did it again one Easter as a punishment for Jake's failure at rope climbing ("Of Human Bonding"). He justified sending his son away on the grounds that Jake had asked for more structure in his life, twisting around what Jake had actually said ("Jake of Hearts"). After Jake graduated, however, Mad Dog paid for Jake to go to Middleton College, either because of the Vietnam War or because he wanted to ensure Jake wouldn't drop out and become a hippie after he left Buxton. ("The Daria Diaries"). "Human Bonding" also revealed Mad Dog didn't even bother to turn up for Jake's wedding, taking the family dog to have its nails clipped instead.
Ruth Morgendorffer first acted unaware of what Mad Dog was like, and then later admitted she did know he was overly harsh. She expressed frustration with how he treated her, giving her a tiny "allowance," and regretted how she'd let him treat Jake, claiming "If I could do it all over, I'd stand up for you against your father. As a matter of fact, I'd do a lot of things differently."
The most telling line about Mad Dog probably comes in “Jake of Hearts,” when Jake tells Daria, “My father always had to point out how I was screwing up, and that screwed me up. I just want to make sure I never make you girls feel that way—less worthwhile or intelligent than your old man.” It appears Jake was successful in this endeavor, though the struggle with his past cost him dearly.
It is generally accepted in Daria fanfic that "Mad Dog" was a career military man, and that he served in either WWII or Korea. Involvement in the Korean War is most often seen because of the time issue, though in 2005 Eichler said "Mad Dog" was viewed as being drafted into WWII. 
He has been portrayed as both a front-line hero and a gung-ho noncom who suffered from his war experiences. While this is not the only view of Jake's father, it is the most widely used explanation for his bad reputation, resulting in Jake Morgendorffer's rants and Ruth Morgendorffer's complaints about him. The Angst Guy once took a different take for "Dark of Hearts" and had him be an Army supply clerk in Arizona, with the nickname coming from somewhere else.
Richard Lobinske offers a sympathetic view of Jake’s father in “Chosin Fate,” the tenth story in the Falling Into College series. While transcribing reporters’ audio recordings from the Korean War, Daria discovers taped interviews with one Corporal Nathan W. Morgendorffer, U.S.M.C, newly arrived at Inchon in 1950, then later recordings as the now-sergeant is being shipped home. Jake’s father was a survivor of the battle at Chosin Reservoir, consumed with hatred for the enemy and probably suffering from post-traumatic stress disorder. He has become bitter and vengeful, missing all the good qualities that earlier recordings with him revealed. In the Daria Von Doom Series, the same author presented another alternative, Maxwell Dean Morgendorffer, a B-25 pilot who flew in the Pacific during WWII and was later mustered out of the service after radiation poisoning during the Bikini Atoll nuclear tests. Ben Grimm considered him to be a hero. In "It's All About Respect," Brother Grimace offers Sergeant Major A.H. Morgendorffer, veteran of World War II, Korea, and Vietnam, a Medal of Honor winner.
A True "Mad Dog"
Some fanfics explore dreadful possibilities for Jake's father as the worst of fathers and human beings.
- First March 2005 interview with Glenn Eichler on DVDaria, with some discussion of Jake's father and his possible wartime involvement.
- “Mad Dog” character description at the Daria Character Database on Glitter Berries