Cyclonus is one Flying Decepticon in the Transformers series.
Transformers: Generation 1
The tech spec from the box art of the Cyclonus toy describes him as a compassionless Decepticon air warrior and saboteur. He has nuclear-powered turbine engines that can propel him at speeds of mach 2 and carries an oxidating laser that fuses an enemy robot's internal mechanisms. It makes no reference to the concept that he was built from another Decepticon by Unicron (as is his usual portrayal in fiction) because at the time of release, this would have been considered a spoiler to the movie.
In the year 2005, after an abortive attack on Autobot City, the evil Decepticons found it necessary to jettison excess mass from their craft to make it back to their homeworld of Cybertron. The injured and dying Decepticons, including the Decepticons' leader, Megatron, were chosen to be ejected into the void of space. When their bodies were recovered by the world-eater, Unicron, they were reborn as deadly new warriors in the service of the recreated Megatron, now known as Galvatron.
In The Transformers: The Movie, having suffered heavy battle damage and abandoned in space, Megatron cuts a deal with Unicron who grants Megatron and his other badly damaged Decepticons new bodies. Due to an animation error, it's ambiguous which one of the damaged Decepticons (either Skywarp or Bombshell) becomes Cyclonus, and the topic is still hotly debated to this day.
Cyclonus is a being whose loyalty to Galvatron is so intense as to exclude most other emotions and interests from his mind. Dedicated to destruction in his leader’s name, Cyclonus transforms into a nuclear-powered multi-turbine-engine jet fighter, capable of space flight and able to expand in size to convey Galvatron. In robot mode, he wields an oxidating laser that fuses the internal mechanisms of its targets, and in vehicle mode is additionally armed with an incendiary bomb rack. Immediately following his creation, Cyclonus served as little more than Galvatron’s personal ship, carrying his leader to Starscream’s coronation, into the battle in Autobot City, and pursuing Autobots on the Planet of Junk, as well as attacking and crippling the shuttle carrying Hot Rod and Kup in the midst of it all. After the destruction of Unicron, however, when Galvatron was lost, Cyclonus came into his own, his loyalty to the Decepticons' absent leader shaping his character in different ways.
Disgusted with how far the leaderless Decepticons had fallen as they eked out a miserable existence on the barren world of Chaar, Cyclonus resolved to locate Galvatron, daring even to enter Unicron's deactivated, disembodied head and replaying its memory banks, tracing Galvatron to the world of Thrull. Unfortunately, once extricated from a pool of the planet's plasma-lava, Galvatron proved to have been driven insane from exposure to it and brutalized Cyclonus and Sweeps before being talked down and returned to his position as leader.
As 2006 settled in, Cyclonus was captured by a Quintesson scientist for study, along with Ultra Magnus, Wreck-Gar and Marissa Faireborn, but when the entire group was sucked through a black hole into a negative universe, they were forced to work together to escape; Cyclonus and Magnus developed a grudging respect for each other as warriors, but vowed that when they met again on the battlefield, it would be as enemies.
Some time later, outnumbered and fleeing from the Aerialbots, Cyclonus and Scourge took refuge inside a vortex, which transported them to the pacifistic, energy-rich planet of Paradron, which had been settled by Cybertronian refugees. Naturally, the duo immediately overthrew the planet and summoned Galvatron and the other Decepticons there; the plan was ultimately foiled by the Autobots, but at the cost of Paradron itself, which was destroyed.
Cyclonus later had his body possessed by the ghost of the deceased Decepticon air commander, Starscream, and went on to clash with Ultra Magnus again aboard the Autobot records asteroid with the fate of Daniel Witwicky in the balance.
When a group of Decepticons, tired of being beaten and abused by the deranged Galvatron, confronted Cyclonus and demanded that he do something about the situation, Cyclonus was so short on options that he accepted the advice of a Quintesson, and tricked Galvatron into accompanying him to the therapeutic mental institution on planet Torkulon. It pained Cyclonus to see his leader so demented as each attempted therapy met with failure, but when the Torkuli attempted to cure Galvatron by removing the damaged part of his mind, thereby leaving him a vegetable, Cyclonus sprang into action, only to be restrained. Patched into the sentient network of the planet, rather than be cured, Galvatron infected the world itself with his madness and freed himself, destroying the planet and decided to turn his attention back to fighting the Autobots, rather than dwell on Cyclonus's deception.
Despite Galvatron’s mental instability, it is evident that he values Cyclonus above all other Decepticons. For example, to escape the clutches of the Trans-Organic Dweller, Galvatron hurls a couple of Sweeps to the beast, which gives both himself and Cyclonus a chance to escape. Although the Dweller eventually captures Cyclonus, Galvatron turns back for a moment and seems to consider going back to help him. Also interesting to point out is that earlier in this episode Galvatron is seen to be taking his frustrations out on Scourge and the Sweeps while leaving Cyclonus completely unscathed.
Later still, he, along with a group of Autobots and Quintessons, were transported into a region of space cordoned off by the Quintessons, landing on the planet Zamojin, where Cyclonus aided the Quintessons in acquiring Perceptor's Universal Emulator, in order to repair their systems and return to normal space.
When the Hate Plague swept the Universe, Cyclonus was infected and took leadership of the afflicted Decepticons, hunting down Galvatron and tracking him and a group of Autobots into the depths of Chaar and infected scientist Jessica Morgan.
In 2007, Cyclonus participated in the massive Decepticon attack on Autobot City which culminated in the theft of the key to the Plasma Energy Chamber. When the chamber was then opened, the energy released blasted the key and a group of Autobots across the galaxy to the planetNebulos, and Cyclonus and Scourge led a team of Decepticons in pursuit, attacking and capturing several of the Autobots. To battle them, the remaining Autobots joined with a group of Nebulan rebels to become "Headmasters" and bested them in combat. Having learned of this fusion of Transformer and Nebulan, the Hive, the evil ruling council of Nebulos, approached the Decepticons to repeat the process, but Cyclonus argued them down to taking the heads of only the animal-mode Decepticons, and merely the weapons of himself and the others. Consequently, Cyclonus became a Targetmaster as his oxidating laser was converted into an exo-suit for the Hive member Nightstick, giving them enhanced targeting abilities and firepower. Reclaiming the chamber key with their superior powers, the Decepticons headed back to Cybertron aboard the giant robotScorponok, where Cyclonus attempted to convince the disgusted Galvatron of the benefits of being bonded to the Nebulans. The Autobots soon arrived back on Cybertron and invaded Scorponok, only to be captured by the Decepticons, and almost executed by Cyclonus before being saved by Spike Witwicky and Fortress Maximus. The Decepticons' plan was subsequently thwarted by Spike and the Nebulans, and they were sent spiralling off through space aboard Scorponok by a tendril of plasma energy.
In the movie and for the majority of the third season, Cyclonus was voiced by actor Roger C. Carmel, who frequently displayed an inability to maintain consistency with the character's voice, noticeably rising and dropping in pitch and tone from episode to episode. When Carmel died towards the end of the series, Jack Angel took over the role of Cyclonus for the final five episodes of the series.