Concept art by Tetsuya Nomura for Final Fantasy VI; while it isn't used in-game, here is where the "Zurvan" name first appears.
Zurvan (
Appearances
Final Fantasy VI
Demon (left) and Zurvan (right)
Demon (きしん, Kishin?), along with the rest of the Warring Triad, is one of the three final bosses in Kefka's Tower before facing Kefka himself. In contrast to the Fiend's durability and the Goddess's status effects, the Demon is much more offensive, using powerful attacks that hit the entire party or inflict Death.
Distinct from the Demon is the Zurvan (ズルワーン, Zuruwān?), an enemy exclusive to the Advance and 2014 Matrix Software releases. The Zurvan can be encountered in the Dragons' Den. It attacks with high-level spells, but due to its low MP and the high cost of its spells, it will quickly run out. Rasp can kill it as the Zurvan dies when its MP reaches 0.
Final Fantasy X-2
Zurvan is an enemy encountered in Besaid Temple, Cactuar Hollow, Cavern of the Stolen Fayth, and Via Infinito. In Sphere Break, Zurvan is also used an alternate name for the Imp family of monsters going back to Final Fantasy X.
Final Fantasy XIV
Zurvan the Demon (鬼神ズルワーン, Kishin Zuruwān?) is one of the Warring Triad, three primals that were defeated by the Allagan Empire and imprisoned for centuries within the Flagship at the heart of the floating continent of Azys Lla. A statue of his likeness adorns the top, along with statues of the other members of the Warring Triad. During the later events of Final Fantasy XIV: Heavensward, his seal begins to weaken and the Warrior of Light must fight him within the depths of 
Containment Bay Z1T9.
Final Fantasy Dimensions II
Demon (三闘神・鬼神, Santōshin Kishin?) was an Eidolon exclusive to the free-to-play version that appeared during a Final Fantasy VI event. It was a fire-elemental signet that when summoned dealt damage to all enemies and allies and inflicted poison for five turns on all enemies.
Pictlogica Final Fantasy
Demon and Zurvan from Final Fantasy VI respectively appear as a boss and enemy.
Final Fantasy Record Keeper
Demon from Final Fantasy VI appears as a boss.
Mobius Final Fantasy
Demon from Final Fantasy VI appeared as a fire-elemental ranger card, and granted the supreme ability Southern Cross.
Final Fantasy Trading Card Game
Etymology
A demon is "an evil spirit or devil, especially one thought to possess a person or act as a tormentor in hell".
In Zurvanism, Zurvan is the god of infinite time (and space) and is Aka Manah ("one", "alone") deity of matter.
In folklore and parapsychology, a poltergeist (German for "noisy ghost") is a type of supernatural being supposedly responsible for physical disturbances, such as loud noises and objects moved around or destroyed. Poltergeists occupy numerous niches in cultural folklore, and have traditionally been described as troublesome spirits who haunt a particular person instead of a specific location.
Kishin is a Japanese word translating to "fierce god". Alternately known as wrathful deities, within East Asian spirituality, kishin are deities based around violence and power but are not necessarily "evil" in nature, representing necessary force, mastery of the natural emotions of anger, and the need to denounce anything unreasonably harmful in nature. They are strong and causes of destruction, but not universally associated with evil as it is often understood in the European/Western framework of moral dualism, and are sometimes interpreted as the angry and provoked sides of normally gentle and merciful deities.