It's called the Mega Gargantua, and if you see it appear on the horizon, it's probably a good time to run.
The Mega Gargantua was created by Minecraft builder Cubehamster, who already has quite a few impressive contraptions under their belt (including a smaller version of this robot) -- but this is something special. It's almost mesmerising to watch each block move to make this huge machine walk, and aim and fire its massive cannons:
Even more impressive is the fact that the robot doesn't utilise any command blocks, which can be used to change different variables of the game and automate some tasks related to redstone machines such as these.
This means that not only does the robot operate fully within the confines of the base rules of Minecraft, but that technically, it could be built on any standard survival level, without resorting to using cheats. Even yours. ... Though that would probably take a lot of time.