Nightbird (G1)
Nightbird was Megatron's precious cuddle-wumpus in Generation 1 continuity
G1
Nightbird was a robotic ninja warrior created by the "famous scientist" Doctor Fujiyama, ostensibly to show how advanced Human technology had become. The Decepticons stole Nightbird, and Bombshell implanted her with an enhanced brain chip which turned the drone-like Nightbird into a true ass-kicking machine. Nightbird broke into the Ark and stole the Autobots' World Energy Chip, which contained the locations of a variety of energy sources, energy sources that the Decepticons would most assuredly use for conquest!
The Autobots were, at first, hampered by their promise to Doctor Fujiyama not to damage or destroy the ninja robot, and found themselves flustered in botched attempts to capture her. Eventually, however, they used an Energy Net which drained Nightbird's power reserves and allowed the Autobots to capture her without serious damage.
Doctor Fujiyama placed Nightbird into cold storage, but apparently didn't see fit to fully deactivate her, as she seems aware of her plight and gives an angry glare as she's stored away.
Trivia
The Chiba Institute of Technology actually built a ninja robot in 2003. Martial arts is used as a benchmark of physical dexterity in robotics.
Apocrypha
Various fan fiction authors have paired Nightbord with Megatron as a potential love interest, based on a few lines of dialog where Megatron declares how impressed he is of her skills. Some fans have deliberate misinterpreted these lines as cries of affection, something that Megatron has never shown being capable of (not that it comes up much in the cartoon).
It's unlikely that Megatron would regard Nightbird as a 'real' robot (IE, a Cybertronian), since she had no personality component to speak of, and showed no signs of a personality or much capability for independant thought - except the sole angry glare as she's being shoved into storage.
Wreckers
In 2025 or so, at which point the retard built an entire army of the damn things. Then they killed Daniel Witwicky and Wheelie, an event which caused Arcee to enter a deep depression. Presumably they all blew up or died or something after that. It was never really explained very well, and nobody fucking cares, anyway. The Nightbirds blew up, that is. Not Arcee. She went off and did other stuff.
Dreamwave
In the Dreamwave comics, a science fiction movie Enter the Nightbird featured Nightbird kicking shiny metal robo-ass.
