Tripods

"How can I describe it? A monstrous tripod, higher than many houses, striding over the young pine trees, and smashing them aside in its career; a walking engine of glittering metal, striding now across the heather; articulate ropes of steel dangling from it, and the clattering tumult of its passage mingling with the riot of the thunder."

- A Martian Technology Report describing these creatures

Tripods from H.G. Wells' novel War of the Worlds (and its adaptions) are giant, bio-mechanic war machines programmed to exterminate the human race and cause astronomical destruction to Earth. Though there are several different incarnations of the infamous tripods, all stories follow the same story. The tripods arrive on Earth, cause destruction, are almost undefeated by the military, and are eventually killed by Earth's viruses.

They are major antagonists in the War of the Worlds franchise and appear in almost all incarnations of the invasion of our planet, and are as well treated as popular culture icons of our world's destruction.

The War of the Worlds (1953)
The Martian war-machines make their first appearance in cinema in the 1935 movie The War of the Worlds, based off the novel with the same name. Instead of containing their usual huge, crab-like appearance as in the books, they are large manta ray-like spaceships with slender yet invisible walking appendages. Armies of these robotic beasts arrive on Earth via a meteorite strike. Despite wielding their greatest powers, skills, and training, the recently arriving US Marines couldn't even put a stop to their rampant massacre. They even drop an atomic bomb on the invaders, but there is no effect due to the machines' impenetrable shieldings. All hope is lost, as Earth is almost forced into evacuation until it is revealed that the tripods and Martians are dying. Not because of waves of heavy war fire, but because of the spread of Earth's deadly viruses that infects, cripples, and kills Earth's now unsuccessful invaders.

The War of the Worlds (2005)
The movie opens up with Ray Ferrier, a divorced crane operator, investigating a mysterious series of violent lightning strikes knifing into the ground. When he arrives at ground zero, he sees that the lightning strikes spawned holes, and what rise from the holes are heat-ray armed, one hundred foot tall tripods ready to cause genocide. The machines then wreak havoc upon Earth, blasting anything in their way and harvesting humans for their tissue, flesh, and blood.

Not even the National Guard armed with families of tanks and fighter jets could stop the tripods, as they gun down any and all types of military support. It is then revealed that their immortality is from their force fields, but Ray eventually destroys the beasts with a group of unpinned grenades. The remaining tripods are killed ironically by simple diseases that wouldn't really harm a human due to the fact they don't have experience or the medicine for it. After all of the invaders or wiped out by diseases or wounds, the Earth is saved and everything returns to normal.

Appearances in other media
The three-legged terrorists, ever since their first appearance, have become stamps and mascots of alien invaders, rivaling that of even the Xenomorphs. As such, they have been referenced and parodied in other pieces of media several times, like in Chicken Little. They were also seen in Scary Movie 4 as they attacked Earth's inhabitants and vaporized everything and everyone with their lasers.