Demons are powerful, evil beings who are often difficult to control and exact a price from anyone who tries to deal with them.  They are usually associated with black mana, and the occasional influence of red mana. Physically, almost all of the demons herewithin are pure manifestations of black mana, without true flesh. Demon is the iconic creature for the color black.  They are all intentionally male. 
Afraid of main stream critcism on the growing game of Magic, both the creature type and the word “Demonic” disappeared in 1995.  For the same reason the pentagram got removed from the Fourth Edition Unholy Strength. For all practical purposes, demons were replaced by Horrors. Demons returned to the game in the fall of 2002 with the release of Onslaught.  WotC had realised that other popular entertainment (from Buffy, the Vampire Slayer to Hellboy) had made demons fashionable. 
Lords of the Pit
A Lord of the Pit is a powerful creature with a gigantic jaw full of razor teeth, two huge wings, and claws. One was summoned by Ravidel at the Dueling Chasm of Golthonor in his battle against Kristina of the Woods, Jared Carthalion, and Altair of Coloni, where the pit lord killed Hoont the spitting slug. Another was summoned by a mage in Kush during fights on Arena. Lords of the pit are so powerful, requiring a constant stream of sacrifices to be kept under control, that even planeswalkers are wary of summoning them. One of them, named Thomil, summoned a lord of the pit in a fight with Worzel and lost his control over it what cost him very long time to recover.
It is unknown whether or not the Lord of the Pit was meant to be the "Devil" of the Magic the Gathering Universe.
Minions of Leshrac
Minions of Leshrac were large, batlike sharp-toothed demonic creatures. Jodah and Jaya Ballard fought one of them in the cave near Tresserhorn. The minions of Leshrac also fought in the battle of Kjeldor and Balduvia with Lim-Dûl.
Large swarms of lesser demons who share a penchant for destroying the land of Dominaria.
Minions of Tevesh Szat
Mold demons are monsters that stand several dozen feet in height and equipped with sharp spikes and teeth made from blue-black mold spores; they are capable of extremely fast regeneration. The shapeshifter Halfdane took the form of a mold demon during his fight with Ayesha Tanaka and Tor Wauki.
Carnifex Demon is a Phyrexian demon who appears as a black skinned humanoid with a large skeletal jaw covering his decaying humanoid face. This demon spreads the contagion around, infecting other creatures with its virus. Ironically, Carnifex himself appears to be infected with the contagion.
Archdemon of Unx
A large demon composed mostly of the tortured visages of the occupants of the necropolis of Unx.
Defiler of Souls
The Defiler of Souls is believed to be the punishment that awaits any who slide into sin by the Bantian people. Bears a striking resemblance to the Prince of Thralls.
Large demons who wear the bones of their victims as accesories. These demons collect the bones (teeth, skulls, etc.) of their prey and sometimes forge the bones into weaponry.
Prince of Thralls
A powerful demon who could command the dead and destroy the living, claiming there is no one he cannot control or command. Given his flavor text, it is implied he may have been the demon who manipulated Sedris into becoming a lich and caused the corruption that is now Grixis. The demon is characterized by its dark blue skin tone and blood stained horns.
Long ago on Innistrad, demons were a threat only whispered about, feared by vampires and humans alike. They were thought to be long dead, or mythical. But then Avacyn and her angels came, and with her presence a new equilibrium was reached. Vampires, werewolves, and the undead were driven back to restore the balance between humanity and the rest of the world's beings. But their retreat opened a space—an opportunity for Innistrad's infernal forces to manifest. The demons and their hosts of devils began to arrive, one by one.
Reaper from the Abyss
Reaper from the Abyss is a powerful, cunning demon, who seduces humans into worshiping him. True to his title, he is a swift and merciless killer.
Withengar is infamous even among demons as being dangerously strong. He fought and succeeded in killing the demon-slayer known as Saint Traft. Soon after, he was trapped within a bone dagger, lying in wait until he could taste the blood of a powerful mage and be set free.
The oni are malevolent spirits of evil and chaos in Kamigawa, most of whom were locked away in the Sokenzan Mountains. Following the outbreak of the Kami War, many were released by their o-bakemono worshipers to bring destruction to the plane. Unlike the demons of most other planes, some oni are created from red mana rather than black. Oni may be recognized by the presence of their third eye. 
Demon hounds exist on Shadowmoor. They are mostly monstrous demons devoid of any intelligence.
Though it is uncertain what the hideous process through which the Cult of Rakdos go through to create half-demons, the result is unique to each one.
A small black demon with an insatiable appetite. They are known for causing panic throughout the streets of Ravnica and consuming the souls of other creatures. However, it is implied that these demons will die quickly if they are unable to find any souls to devour.
Several types of demons exist in Tarkir, mostly associated with the Sultai Brood and the Mardu Horde. The most unique are the Rakshasa, demonic tiger-like cats, with which the Sultai have made pacts for power.
Theros demons are souls long consumed by hatred and other negative emotions, eventually corrupted by black mana in an astonishingly quick process. With some effort, some fly back to the world of the living from the Underworld, leading "second lives" in a myrad of ways, usually as yet another form of wild monster.
Various demons exist on Zendikar, most of which possess incredible and rather destructive powers. The most notable demon on Zendikar is Ob Nixilis, a fallen planeswalker who lost his spark and seeks revenge against the land that corrupted him. Like most other demons on other planes, these demons covet power and wish to take the land's energy for themselves.
The Hellcarver are demons who demand litterally ALL physical materials for their services, once the pact is made, one must sacrifice everything for them. The price is great, but the reward they offer is irresistable.
Demons who hunt down and kill angels where they roam and collect the remains as trophies to hang in their homes.
Abyssal Persecutor A ruthless and efficient demon easily summoned by planeswalkers. The drawback is that they enjoy abusing their victims so much that they use their dark arts to make their victims immortal.
A demon similar to Ob Nixilis in that they covet power and domination. They fly around, inflicting plague and torment upon everyone around them. It is also interesting to note that pestilence demons seem to bear a traditional, goatlike appearance.
Demons of Death's Gate
A powerful demon of unknown origin that can be released through the offering of marrow and sinew and blood. It is characterized by its monstrous gapping maw and colossal chiropteran wings.
Demons possessing a menacing grin and pale skin, the Xathrid are powerful demons known for making contracts with people in exchange for power. However, the only currency they accept is flesh and blood, much to the horror of their clients and cultists.
- Griselbrand, one of four influential archdemons who claimed ownership over the soul of Liliana Vess. On Innistrad, he rose in power to eclipse all other demons. Griselbrand was locked into the Helvault, together with Avacyn. After their escape, he was killed by Liliana.
- Shilgengar, the demon which preyed on Edgar Markov's ambition and whispered secrets that would point the way to the creation of vampires on Innistrad.
- Withengar, the killer of Saint Traft
- Archdemon of Greed
- The All-Consuming Oni of Chaos
- Kagemaro, First to Suffer was the first human to become so evil that his spirit became an oni or "demon spirit". His strength is determined by the number of cards in his controller's hand. He can also be sacrificed to cause a sometimes fatal reduction in strength of all creatures in play.
- Kuro, Pitlord was a powerful oni who took many Nezumi and others as his minions.  
- Kyoki, Sanity's Eclipse draws on the power of spirit magic to eat away at an opponent's mind.
- Seizan, Perverter of Truth. A powerful, evil oni known for its ability to deceive.
- Shimatsu the Bloodcloaked. An oni overlord.
- Yukora, the Prisoner, imprisoned by 99 monks with a magic spell.
- Urami, a legendary oni caged underground for centuries. 
- Abcal-dro was a Phyrexian Inner Circle member who oversaw Belbe's creation, implanted her lens and used it to oversee her selection of the new evincar.
- Croag was a powerful Inner Circle member who worked alongside Davvol on Rath. He was badly wounded by Kreig.
- Kirril was a close servant of Yawgmoth who brought Crovax to Phyrexia for compleation.
- Ob Nixilis, a fallen planeswalker trapped by Zendikar's wild mana. It is unknown if he was a demon before his fall or not.
Grand Creature Type Update
- Infernal-Denizen (Infernal Denizen)
- Minor Demon (Boris Devilboon)
- Mold-Demon (Mold Demon)
- Wretched (The Wretched)
Demon tokens are produced by:
- Boris Devilboon (1/1 black and red Demon "named Minor Demon")
- Demonic Rising (5/5 black Demon with flying)
- Promise of Power (X/X black Demon with flying)
- Skirsdag High Priest (5/5 black Demon with flying)
- Tomb of Urami (legendary 5/5 black Demon Spirit with flying "named Urami") 
- Withengar Unbound has the highest power and toughness of any demon.
- Brady Dommermuth. (July 05, 2004.) "Demons! Demons! D3m0nZ!!1!", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Mark Rosewater. (March 20, 2012.) "Marquee creatures for the colors", Blogatog, Tumblr.
- Mark Rosewater. (August 01, 2014.) "The fact that all angels are female has come up a few times here. I noticed that all demons happen to be male. Is that intentional too?", Blogatog, Tumblr.
- Mark Rosewater. (October 17, 2011.) "The Walking Undead", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (September 17, 2002.) "Sketches: Grinning Demon", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Mark Rosewater. (July 05, 2004.) "Where Have All The Demons Gone?", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (July 07, 2004.) "Demonic Arcana", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (March 22, 2005.) "Third Eye of the Oni", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (July 26, 2005.) "Yawgmoth's Demon", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Jay Moldenhauer-Salazar (January 05, 2005), "A Servant’s Mission", magicthegathering.com. Wizards of the Coast
- Magic Arcana. (June 16, 2005.) "Human to Rat", Daily MTG, magicthegathering.com, Wizards of the Coast.
- Magic Arcana. (June 14, 2005.) "Urami token art", Daily MTG, magicthegathering.com, Wizards of the Coast.