In "Terminator 2: Judgment Day," the shape-shifting liquid-metal villain is the T-1000. Portrayed by Robert Patrick, it is an advanced Terminator sent by Skynet to travel back in time and kill John Connor as a child.
Unlike the T-800, whose metal endoskeleton is covered with living tissue, the T-1000 is made from a fictional liquid metal called mimetic polyalloy. This lets it imitate people and objects, flatten itself, pass through barriers, and form blades from its own arms.
The T-1000 usually appears as an LAPD officer, allowing it to pursue John while remaining deceptively human. The T-X belongs to a later film, Terminator 3: Rise of the Machines, while the T-3000 appears in Terminator Genisys. The designation T-800 therefore describes the protector in the sequel, not its principal villain.