These are the types of Metroids that appear in Return of Samus. There are 8 life stages, with 7 present in the game.
Metroid egg[edit]
The Metroid egg is found when you defeat the final boss. Soon, it'll hatch and give birth to something new.
Metroid infant[edit]
This is the last metroid, which calls you its mother. It will not harm you in anyway. It will appear after the metroid egg hatches. This type of metroid turns into a larval Metroid, which you encountered in the first game and at the end.
Metroid[edit]
This Metroid is the Metroid you saw in the first game. After defeating all types of Metroids, 8 will appear to protect the Queen.
Alpha Metroid[edit]
These are evolved, adolescent Metroids. There are 15 to kill in SR388. These metroids ram at you, inflicting damage onto Samus. Shoot them with 5 missiles on their underside.
Gamma Metroid[edit]
These metroids are mature, more powerful Metroids, with 16 to deal with. Now, not only do they ram at you, but shoot bolts of lightning bolts. These deflect your missiles, and give damage. It needs more missiles in order to kill. 10 will do the job.
Zeta Metroid[edit]
More powerful than before, Zeta Metroids are now able to stand up and move quicker than ever. The lightning bolt is replaced with acid spit. Attack it from front and behind with 20 missiles. However, you only find three of these kinds.
Omega Metroid[edit]
The penultimate (or in some games, final) mutation of a Metroid, these things are high powered. Now standing straight, they are slower than their Zeta counterparts, but deal more damage than ever. It'll shoot projectiles, slash at you, and ram you. Sometimes they'll float around. Use the Screw Attack to deal damage. It takes a whopping 40 missiles, but fortunately can be hit with the Ice Beam. Luckily, you'll only find four.
Queen Metroid[edit]
The biggest and baddest of the bunch, you'll find this Metroid as the final boss. Kill her to destroy the production of Metroids. Defeating her will need a tremendous 150 missiles, or, if you got the energy to do it, bombs.
Upon entering, she'll immediately attack, so jump quickly. Then the battle begins. She'll spurt out blobs of acid, which will follow you. Use missiles or the Screw Attack to stop them. You'll find that the more she gets hit, the faster she becomes.
There are two strategies to defeat her.
Missile method[edit]
This is an easy method, more for those wanting to have their energy consumed less. However, this method needs 150 missiles. Make sure you get all the missile expansions.
When the Queen opens her mouth, shoot a missile. Then, shoot more, and she'll continue to be harmed. Continue to repeat this, various times until she is defeated.
Bomb method[edit]
This is a more sophisticated method, for those who have collected their Energy Tanks. This method makes it much easier, but deals more damage.
As per normal, shoot a missile in to her mouth. However, instead of shooting missiles, enter morph ball mode. Then, jump into her mouth (or use bombs to jump up if you didn't get the spring ball upgrade), and roll through into her stomach. Be quick, for you will take damage. Once in her stomach, drop bombs. Then, exit, and the Queen will be harmed. Repeat 10 times, and she will be defeated.