Kraid is a dinosaur-like alien that appears in Metroid, Super Metroid, and Metroid: Zero Mission. In all three games, he is one of the two (four in Super Metroid) monsters required to be defeated in order to reach Mother Brain.
In Metroid (NES)
In the very first game he, 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, his physical appearance changes dramatically. Kraid becomes a giant dinosaur-like monster with three eyes, and three holes on his stomach which can shoot out t. rex-like arms.
MetroidKraid, alongside Ridley, is one of 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 boomarang-shaped spikes from his back. Upon defeating Kraid, Samus is rewarded with a 75 missile ammo capacity.
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.
Super Smash Bros. Melee
Kraid appears as a background character in the Super Smash Bros. Melee stage "Brinstar Depths". He will pop-up from lava, roar and then whack the rock with his claws, causing it to rotate to make the combatants fall into the lava. There is also a trophy of Kraid that is unlocked alongside "Brinstar Depths".
Metroid: Zero Mission
In Metroid: Zero Mission; much like Ridley, Kraid is one of the bosses that must to be fought in order to processed further into the game. He fights pretty much the same way he did in Super Metroid, retconning his original appearance in the original 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, 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. Instead, Kraid makes a later cameo.
He is the boss in three missions. 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 its 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 even summons Zebesians and Cyclons to help him. After three rounds, Kraid's weak spot explodes, destroying him.