The T-1000000, also referred to as the T-Mega, was a gigantic spider-like version of the T-1000. It defended Skynet's central core from attack. Like the T-1000, it is entirely made of mimetic polyalloy.
The T-1000000 is apparently not capable of mimicking subjects as the Series 1000 Terminator can, but instead metamorphosises from an apparently solid part of Skynet's central core to a default arachnid configuration capable of forming stabbing, slashing weapons from each of its eight legs.
The only known T-1000000 was destroyed when Skynet's central core was destroyed.