D-Rex is the boss of the third section of Sigma Palace in Mega Man X. It was supposed to be a destructive weapon with about 30 meters length based on the T-Rex (hence the name), but by the time that Sigma's hideout was invaded by X, only the head was completed, so it was converted into a tank-like Mechaniloid to be used against him.
Its top can detach from the bottom, hovering in the air, and both try to crush and seriously damage X. When either section collides with a wall, it can daze X and knock him off. Both sections can also charge up massive electron spheres that inflict devastating damage. Its main weakness is the Boomerang Cutter.
In Mega Man X: Command Mission, the D-Rex is a common, but powerful, enemy that appears in the Vanallia Desert. This D-Rex is a brownish-grey and is more dinosaur-like in appearance. D-Rex attacks with shots, Heterostorm and releases explosive miniatures called Mini Rex. Mini Rex can be prevented if D-Rex's jaw is broken with at least 11 C-type attacks.
Run away in his third turn onwards after being damaged by 11 C-type attacks. Trickstar and Stealth Mode take damage of Mini Rex.
D-Shark is a weaker and more colorful version of D-Rex found at the Tianna Camp in Mega Man X: Command Mission. D-Shark attacks with Ultra Blizzard and releases explosive miniatures. Mini Shark can be prevented if D-Shark's jaw is broken with at least 3 C-type attacks. Trickstar and Stealth Mode take damage of Mini Shark.