"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 said in a 2005 interview he was supposed to have been in the U.S. Army as part of the draft.
While he never appears, his 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; that his father would basically ignore Jake when he got hurt and tell him to man up ("Monster"); 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") and thus himself rarely showed emotion ("Jake of Hearts"). The end result was that Jake was still wetting the bed at age 15 ("The Teachings of Don Jake").
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 "The Daria Hunter", but spun it as something Jake wanted to do as he wanted 'structure', twisting what the boy actually said ("Jake of Hearts") - 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"). Jake sent numerous letters home, first plaintive and begging to come home, which "Mad Dog" ignored; and then bitter, saying he didn't want to flunk or run away so his father couldn't hold that over him. ("Diaries")
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, as he was threatening to do. ("The Daria Diaries").
In "Of Human Bonding", Jake says his father took the family dog to have its nails clipped instead of go to his wedding.
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" to live off of, 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.
(Daria helped Jake get out of his funk by pointing out her grandfather had died before reaching his early forties and thus, Jake was better than him at not being dead.)