For typical enemies see Super Metroid/Enemies. Bosses are listed in order of appearance.
Mini Bosses, like Bosses, serve to hinder Samus' quest in exploration of Zebes; most often with a power-up nearby (before or after the fight). The main difference between Mini Bosses and Bosses is the difficulty, size and effect on the game.
Ridley (Space Station Ceres)
This battle isn't a battle that you can win, but you can get a slightly altered scene depending on what you do. If you let your health drop below 30, Ridley decides to get out of the battle. He flies into the screen, knocking Samus into the wall if he knocks into her. However, if you shoot Ridley enough times before your health drops below 30 (about 100 shots), he'll drop the Metroid hatchling, grab it again and fly up and into the screen.
Both of these will trigger the countdown sequence, giving you one minute to get off the station.
A Chozo statue that comes to life and attacks Samus after she takes the morph ball bombs. It attacks by walking across the room and slashing Samus, and spewing bombs on the floor when it's against the wall. Its weak point is its chest, shoot it 8 times with missiles or use normal beam attacks to destroy it.
This giant spore will swing back and forth trying to hit you while the plants at the top of the screen shoot spores at you. The simplest way to defeat it is to equip missiles or Super missiles, then go into a morph ball. He can't hit you in the morph ball form, so just focus on dodging the spores. Eventually, he will stop at one side of the screen and open his mouth. Take this chance to shoot a missile into the mouth. You can get two hits in at once if you shoot fast enough. Around six missiles should take him down.
If you managed to get the Super Missiles earlier than usual, it should be 3 times quicker.
Crocomire is the Norfair miniboss, available to fight once you've acquired the Super Missiles. When you drop down into his lair, Crocomire will be standing to the right, and will begin to approach you. If you don't act fast, he'll end up pushing you all the way to the end of the room where you'll be impaled on the spikes on the wall. Its only weakness is to shoot it in the mouth. If you can hit it with more powerful weapons (Super Missiles) you'll knock it back slightly. Use them to hold him back while you pummel him with everything you have, until you send him back far enough that he falls into a lava pit. If you run out of missiles you'll have to rely on charged shots. Don't use power bombs, as this will make him angry and he will charge forward.
A sea serpent found in Maridia that swims in and out of holes in the wall trying to hit Samus. Its weak point is its head. As it is damaged it will start to swim faster. Sometimes it will stick its head out of one of the holes and shoot energy balls across the room.
A stronger version of the first torizo, found much later in the game in Ridley's lair in Norfair. Its weak point is still on its chest. Its attacks are mostly the same but much faster and stronger, and it will also rapidly shoot energy waves across the room occasionally. Normal missiles do not affect the Torizo; he simply dodges them. If you use a super missile, he will catch it and get rid of it (however, while he is holding a super missile, he is vulnerable to missile / super missile attacks). Charged plasma beam shots are the easiest and most effective way to damage him. Defeating the Torizo earns Samus the screw attack.
The four bosses, Kraid, Phantoon, Draygon, and Ridley must be defeated before the way to Mother Brain is opened (Tourian).
By far the largest of the Space Pirate crew. Kraid has been used in some of the Metroid games, but not as many as Ridley. Kraid attacks by throwing his razor sharp claws at you, or by the strange spike-like things that protrude from his 3 belly buttons. You can only hurt Kraid by hitting him in the mouth. Kraid and its strange mini-version are very easy bosses; you shouldn't have anything to fear here.
A ghostly squid-like being that haunts the wrecked ship. It attacks by shooting blue flames in various patterns. Occasionally it will open its eye, which is its weak point. Shoot it with missiles or a charged beam shot, eventually this will kill it. If its eye is shot with a super missile it will get angry and shoot several waves of flame across the room, so be careful. Defeating the boss restores power to the wrecked ship.
A large, green levitating aquatic creature that fights Samus in the bowels of maridia. When Samus enters its lair the walls are lined with gun turrets and its babies are floating aimlessy, but the actual boss is nowhere to be seen. It soon flies in however, and attacks mostly by spewing gunk at Samus that will slow her down, and then grabbing her while hitting her with its tail. There are two ways to defeat this beast: Samus can either a) shoot its belly with missiles while it flies around, slowly turning its belly brown and eventually killing it, or b) destroy the gun turrets on the walls, and while Draygon is holding her, shoot one with the grappling beam to electrocute the beast and kill it quickly. Method (b) is much quicker and easier, but it may result in more damage being done to Samus. Either way, the death of Draygon earns Samus the space jump.
A well known boss of the Metroid series and Mother Brain's number one henchman. Throughout almost every boss battle with Ridley, most of the attacks consist of fireballs and just ramming into you or picking you up and throwing you by stabbing you with its tail. Shoot with missiles then super missiles then a charged plasma beam.
The leader of the Space Pirates and the final boss. The first phase of the fight is much like it was in the original Metroid, where Samus makes her way across the acid- filled chamber and fires away at mother brain while gun turret defense systems fire back. Samus seemingly destroys the helpless brain again, but it soon rises up from the ground, now connected to a robotic T-rex-like body. Her attacks consist of shooting blue energy rings out of her mouth, lighting from her eyeballs, exploding rock bombs she drops out of her mouth, and a stream of fire from her hands. Also, the brain itself will glow and fire a large multi-colored beam that deals significant damage and drags upward, making it almost undodgeable. During the middle of the fight, you'll get hit by the beam and you won't be able to move at all. When Mother Brain charges up the killing shot, the baby Metroid Samus found in Metroid 2 (now much larger due to being exposed to radiation) sucks the life out of Mother Brain and gives that energy to Samus, replenishing her health. Whilst that happens, Mother Brain reanimates and starts shooting the Metroid, and ends up killing it, but in doing so, Samus gets the Hyper Beam that will make quick work of Mother Brain.