Kraid is the secondary antagonist in the Metroid series. He is a dinosaur-like alien which is a Space Pirate general and one of the two (four in Super Metroid) monsters required to defeat in order to reach Mother Brain.
In the very first game, Kraid, much like Ridley, was just merely a miniboss and comparable in size to Samus Aran, maybe even smaller.
In Super Metroid/Metroid: Zero Mission
In both Super Metroid, and Metroid: Zero Mission, Kraid's physical appearance changes dramatically. He becomes a giant dinosaur-like monster with three eyes, and three holes on his stomach which can shoot out t. rex-like arms.
Metroid/Metroid: Zero Mission
Kraid and Ridley are the two minibosses that must be fought in order to make it to Mother Brain. Kraid fights by shooting spikes out of holes in his stomach and boomerang-shaped spikes from his back. Upon defeating Kraid, Samus is rewarded with a 75 missile ammo capacity.
In Metroid: Zero Mission, Kraid fights similarly to he did in Super Metroid, retconning his design from Super Metroid and the location of his battle from the original Metroid game. Once Samus deals the final blow, Kraid partially sinks into the ground and lets out a loud shriek before exploding, allowing Samus to gain the Speed Booster.
Kraid fights with razor-sharp claws and shoots out T. rex-like arms out of his stomach. His main weaknesses are his third eye and his mouth. When Samus shoots at the eye, Kraid's mouth opens for a second. Samus then shoots Kraid in the mouth with a Charged Beam, Missile, or Super Missile, injuring Kraid. She does this several times until Kraid is defeated, allowing her to gain the Varia Suit to venture into Norfair.
Kraid, known amongst fans as "Meta Kraid" in a manner similar to the game's Meta Ridley, was supposed to be a boss in the first Metroid Prime game, but was cut and replaced with the Omega Pirate. Although despite being scrapped, Kraid is mentioned in the manual of the game. Instead, Kraid makes a later cameo.
Super Smash Bros.
Super Smash Bros. Melee
Kraid appears as a background character in the Super Smash Bros. Melee stage "Brinstar Depths", unlocked by playing 50 Vs. Mode matches. He will pop-up from lava, roar, and then whack the rocky stage with his claws, causing it to rotate to make the combatants fall into the lava below. There is also a trophy of Kraid that is unlocked alongside "Brinstar Depths".
Super Smash Bros. Brawl
Kraid appears as a Sticker that grants the player additional power for all head-based attacks.
Super Smash Bros. Ultimate
Kraid returns as a hazard for the "Brinstar Depths" stage, acting exactly like he did in Melee. His appearance is based on his Zero Mission appearance. He will only appear in the regular unaltered stage and not its Battlefield nor Ω form.
In "World of Light", Kraid is one of the 24 Master Spirits who runs a Dojo after being defeated. His Spirit Battle is a Stamina battle against a giant King K. Rool. When Kraid's Dojo is unlocked, he will teach four Spirits the Tank Style, which restricts movement and jump velocity for stronger offense and defense power.
Kraid is a boss in three missions of the Nintendo Land minigame "Metroid Blast". He occupies the center of the Norfair-themed stage and has a 360 degree of range. Unlike the rest of the enemies, Kraid is stationary and can only rotate on his own axis. To defeat Kraid, the red spots on his body must be hit, causing him to open his mouth, where the player must fire three charged missiles into his mouth to damage the massive spot in his transparent belly. Every time his weak spot is hit thrice, he rises higher from the lava and summons Zebesians and Cyclons to help him. After three rounds, Kraid's weak spot explodes, destroying him.
Kraid's roars in Zero Mission are stock roars used by the Tyrannosaurus rex from the 1959 film The Land Unknown, and by King Kong from the 1976 remake of King Kong.
In Super Metroid, Kraid shares his boss fight background music with Crocomire and Phantoon.