Megaman X: Throwing Back Towards the Original Megaman

Aside from the obvious fact that Megaman X is in fact Dr. Light's greatest creation (and could be a revamped Megaman, rebooted and rebuilt for future use), the Megaman X series tries to throw back itself towards the classic series in spite of its tone change from lighter to darker. Here's some I can name:

Megaman X

If you have noticed in Megaman X, you will end up noticing some of the bosses are throwbacks to classic Megaman bosses namely:

• Flame Mammoth, Chill Penguin, Electric Sparkmandrill, and Boomer Kwanger might be references to Fireman, Iceman, Elecman, and Cutman (all from Megaman 1).
• Boomer Kwanger may also reference Quickman in Megaman 2.
• Armored Armadillo's stage may remind you of Gutman's stage. There are also the Degravers which are stand-ins for the Picklemen in the original game.
• Sigma as the main villain has these throwbacks from Dr. Wily in this way. Remember when Dr. Wily reprogrammed the robot masters from Megaman 1 to work for him? Well, Sigma recruited several Maverick Hunters to start his quest for rebellion against humanity.
• Vile's appearance may remind some of the fourth Darkman robot in Megaman 5. The references are there such as before fighting Darkman - Megaman does get severely injured. In Megaman X, X gets severely injured 
• The fortress stage's boss rematches don't work like the capsules. Instead, you get to face them in nearly the same way as the first Megaman game. 

Megaman X2 has these references to the original series:

• Serges looks like a robotic version of Dr. Wily. When you fight him in the fortress - he uses a giant machine instead of fighting with his own power before you destroy him. The Japanese dialogue (which was changed in the American) even hints that he could be Dr. Wily in a robotic body.
• Bubble Crab, Flame Stag, Wheel Gator, Crystal Snail, and Overdrive Ostrich are references to Bubbleman, Heatman, Metalman, Flashman (but having a different version of a time-stopping weapon), and Quickman. 
• More interestingly, Bubble Crab is weak to Wheel Gator's weapon, Flame Stag is weak to Bubbleman's weapon and Overdrive Ostrich is weak to Crystal Snail's weapon. Crystal Snail's weapon slows things down when charged. 
• Crystal Snail's stage may remind some of Crystal Man's stage in Megaman 5. 
• Magna Centipede is a reference to Magnetman. His weapon also works like Crashman's. 
• Wire Sponge's stage may be a reference to Woodman's stage. 
• Morph Moth's stage may be a reference to Junkman's stage. 

Megaman X3 has these references to the original series:

• Tunnel Rhino is a reference to Drillman in Megaman 4.
• Volt Catfish may be a reference to either Elecman in Megaman 1 or Sparkman in Megaman 3.
• Crush Crawfish may be a reference to Cutman in Megaman 2.
• Gravity Beetle is a reference to Gravity Man in Megaman 5. The charged version of the weapon may remind you of the Gravity Hold too.
• Sigma like Dr. Wily manages to double-cross a good scientist. Sigma manipulates Dr. Doppler into creating the Kaiser Sigma body just as Dr. Wily duped Dr. Light into creating the Gamma Robot. 
• Zero also plays a role in saving Megaman X like Protoman saved Megaman. 

Megaman X4 has these references to the original series:

• Zero is also revealed to be Dr. Wily's final creation when using Zero.
• Split Mushroom splits into two like Geminiman in Megaman 3.
• Slash Beast may be another reference to Slashman in Megaman 7.
• Jet Stingray's stage takes place in the ocean with the Land Chaser. It may be a reference to Waveman's stage the part where Megaman must use a jetski and can't use special weapons.
• Jet Stingray's weapon is a reference to Snakeman's weapon in Megaman 3.
• Storm Owl may be a reference to Airman in Megaman 2.
• The Repliforce got manipulated into the war just as Dr. Cossack was manipulated. In this case, Sigma was a lot more sinister about it.  

Megaman X5 has these references to the original series:

• Izzy Glow may be a reference to Brightman in Megaman 4 except his weapon isn't a flash type.
• Dark Dizzy may be a reference to Shademan in Megaman 7, both being bat-like bosses who can recover some health by sucking the player's health.
• Spike Rosered is a reference to Plantman in Megaman 6 except he doesn't use a plant barrier and that he can clone himself. 
• The Skiver's weakness to a flash-based weapon may remind you of Quickman's weakness in Megaman 2.
• Dynamo as an arrogant menace may also be a reference to Bass' arrogance. Unlike Bass, Dynamo doesn't rebel against Sigma.
• The Falcon Armor and Gaea Armor seem to be tributes to the two power modes from Megaman 6 namely Jet Megaman and Power Megaman. However, you can't switch armors presumably due to memory space.
• The first fortress stage has laser cannons that you can stop using Dark Dizzy's weapon or you need to be extra quick to survive those.
• Somehow Sigma was able to secure the new kind of Grey Devil which is more terrifying than the rest of the Devil robots in the Megaman series. When its health drops low, it forms into the form of a Wily Machine.
• When fighting either Megaman or Zero - the Dr. Wily logo appears.
• Sigma's final form resembles Gamma from Megaman 3. Also, he makes a reference to Dr. Wily when fighting Megaman X. This may be nothing more than having contact with Dr. Wily in a figurative sense since the latter had already died decades ago, while he had downloaded Zero's data for himself.
• The threat of the Sigma Virus is a reference to the Evil Energy in Megaman 8.

Megaman X6 has these references to the original series:

• Rainy Turtloid's stage would rain like Toadman's stage in Megaman 4 except it corrodes the player's health as well. The Inami Temple may also be a reference to Yamato Man's stage from Megaman 6.
• Ground Scaravitch is a reference to Groundman from Megaman and Bass. The Central Museum may be a reference to Knightman's stage in Megaman 6. 
• Metal Shark Player is a reference to Junkman from Megaman 7. The rising and crushing boulders may remind one of Knightman's stage in Megaman 6.
• Blizzard Wolfgang may be another reference to Blizzardman from Megaman 6. Even his weakness is fire-based.
• Blaze Heatnix's stage is in a fuel manufacturing plant similar to Flameman from Megaman 6. Both their weapons beat ice-themed bosses.
• Commander Yammark's stage is in the Amazon area. Plantman's stage from Megaman 6 Both bosses also use barrier-type weapons. 
• The Shadow Armor is an enhanced version of the Power Megaman mode from Megaman 6. 
• The Blade Armor, though not capable of flight, can be used to reach some hard-to-reach areas like the Jet Megaman mode from Megaman 6. Why the Falcon Armor in its damaged state (while being able to air dash and charge weapons now) got integrated into this game is quite weird.
• The Eye Blobs may be a reference to the first fortress boss, Rounder II, in Megaman 6 except the boss battle is much harder. Using the Shadow Armor for beginners may be a reference to Power Megaman in defeating that same boss. 
• Zero tries to pull out a Protoman impression similar to that of Megaman 8's ending. Knowing Zero was created by Dr. Wily, it's likely he has bits and pieces of Protoman's personality. Also, finding Zero may be a reference to finding Protoman in Megaman 6. 

Megaman X7 has these references to the original series:

• Axl as a new character is a nuisance to Megaman X and to Zero. Like Bass, he's got the thirst for fighting and love for battle.  
• Soldier Stoneking is a reference to Stoneman in Megaman 5.
• Wind Crowfang is a reference to Windman in Megaman 6 and Tenguman in Megaman 8.
• Flame Hyenard is a reference to Flameman in Megaman 6.
• Splash Warfly is a reference to Waveman in Megaman 5.
• Ride Boarski is a reference to Turboman from Megaman 7.

Megaman X8 has these references to the original series:

• Optic Sunflower is a reference to Plantman in Megaman 6. His stage may remind one of Astroman's stage in Megaman 8. Also in his stage, there is a special code to fight Cutman way back in Megaman 1.
• Gravity Antonion is a reference to Gravityman in Megaman 5.
• Burn Rooster's being weak to Avalanche Yeti's is a reference to Fireman being weak to Iceman's weapon.
• Burn Rooster is a reference to Burnerman in Megaman and Bass.
• Bamboo Pandamonium may be a reference to Woodman in Megaman 2, both weak to fire weapons.
• Vile trolling the player felt like Protoman trolling Megaman in Megaman 3.
• Fighting a fake Sigma in this game may be a reference to Dr. Wily using a fake Wily in Megaman 3.
• Lumine's turn heel may be a reference to Sunstar being the final boss instead of Wily in Megaman World 5.

Megaman X Command Mission has these references to the original series:

• Zero's Absolute form is a reference to Bass with Treble Boost. I wanted this form for Axl instead!
• Incentas is a reference to Pharaohman from Megaman 4.
• A Giant Metool is one of the enemies in the mines where you will fight Incentas. It throws back to giant Metools in the classic Megaman game.