D-Rex is the boss of the third section of Sigma Palace in Mega Man X. It was supposed to be a large Mechaniloid based on the T-Rex (hence the name), but by the time the fortress was discovered by Zero, only the head was completed, forcing Sigma to convert it into a tank.
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.