Mega Man X3/Bit, Byte, and Vile

Cutscene
After Mega Man X defeats two Mavericks, the following cutscene will occur.

After this cutscene, Bit can appear as a boss in the middle of stages. Byte can start appearing after you've beaten 5 Mavericks. Whether they show up or not is random, so you'll need to make sure you fight and defeat them both before you unlock the final stages if you want to take them down. Furthermore, they will definitely reappear in a later stage if Mega Man X is defeated during a battle versus any one of them. If you're running out of Maverick stages to fight them in, you may want to restart from your password whenever you reach an empty room or die to them.

At the same time, three teleport hatches leading to Vile's stage will appear in Blizzard Buffalo's, Volt Catfish's, and Crush Crawfish's stages. You can access it from any of these 3 stages as long as any Maverick is still alive, not necessarily one of these three. They will disappear after you fight the final Maverick though, so don't forget to go back and fight him.

Bit

 * Original name: ヴァジュリーラＦＦ (Vajurīra FF)
 * Location: third stage (or later)
 * HP: 32
 * Attack: 4 damage with fireballs, 3 damage with beam saber, 1 damage on contact
 * Weaknesses: Frost Shield: 2 damage, final blow destroys, Triad Thunder: 2 damage, final blow destroys, Charged Shot: 2 damage, Cross-Charge: 2 damage

If you defeat Bit with Frost Shield or Triad Thunder and Byte with their weakness they will not reappear as boss of the first final stage. A different boss will be there instead.

Bit's main attack is to charge at you with his saber drawn. He starts by jumping, and as he falls he stops when he is aligned with you, and charges forward. The beam saber blocks most attacks from in front of him at this time, so the best time to hit him is from behind or below during this attack.

His next attack is a fireball that homes in on you. You should be able to avoid it if you keep moving. The final attack he has is throwing rings at you that will hold you in place. If they hit the wall, they should fall and continue to roam back and forth on the ground for a short while before disappearing. If you land in one you should tap left and right rapidly to break out, but Bit'll likely hit you before you're able to. Try to avoid getting caught in them in the first place.

Frost shield is awkward to use due to the beam saber. You either need to shoot at him and then jump at just the right time so that the saber goes above the shot and he gets damaged, shoot as he is coming down from the jump for the attack, but this leaves you with little time to avoid it, or you need to have the shot hit the wall and fall down as a hazard and it'll be just below the saber at the lowest his charge attack can go. Charging Frost Shield is almost worthless for this fight unless you get lucky as the beam saber will break it.

Triad Thunder is a little bit easier because you just need to fire the shot when it will hit him from behind or below while you're dodging it. The charged Triad Thunder isn't much good either, you'll usually get hit trying to perform it.

Byte

 * Original name: マンダレーラＢＢ (Mandarēra BB)
 * Location: sixth stage (or later)
 * Weakness: Tornado Fang (T. Rhino's) or Ray Splasher (N. Tiger's)

If you defeat both him and Bit with the weapons stated here, they will not reappear as boss of the first final stage. A different boss will be there, instead of them.

Vile

 * Original name: ヴァヴァ (Vava)
 * Location: teleport hatches in three stages: B. Buffalo's, V. Catfish's and C. Crawfish's;
 * Weakness: Spinning Blade (C. Crawfish's) or Ray Splasher (N. Tiger's)

The three teleport hatches will stay in the stages even if the boss is defeated. Therefore, you can and should visit the Vile's stage after you obtained at least one of the weapons stated above. The hatches disappear if you defeat all eight Mavericks.

If you defeat Vile with the weapons stated here (at least the final blow), he will not reappear as mini-boss of the second final stage. A different boss will be there, instead of him, and you can then obtain Zero's Saber.