VGFacts

Warning: this contains spoilers for both Undertale and Chapter 2 of Deltarune.

In Chapter 2 of Deltarune, the player encounters the character Spamton, who offers two different sidequests that can be undertaken (the Keygen fetch quest and what is interchangeably known as the Weird/SnowGrave/No Mercy/Genocide/Pipis Route), both of which culminate in a fight against him as Spamton NEO. Throughout his role in the game, Spamton draws a number of parallels to Undertale's Mettaton, with this myriad of references being detailed below. Note that due to the scope of this topic, this is going to be a much longer post than usual.

Spamton NEO and Mettaton NEO
The first and most obvious parallel Deltarune draws between the two characters is their names, particularly the presence of similarly-designed NEO forms:



What's more, speaking with Swatch after defeating Spamton NEO and selecting the "Basement robot" option reveals that his form was originally designed by a Lightner-- heavily implied to be Deltarune's equivalent of Mettaton (given the robot's design and his dialogue in the epilogue about being "a nobody")-- whose aspirations never came to fruition.



Spamton NEO's fight
In addition to these features, various elements of Spamton NEO's fight hark back to the multiple battles against Mettaton. For ease of reference, both the Mettaton/Mettaton EX fight and the "normal route" fight against Spamton (which draw the highest amount of similarities) will be shown below:



 

From these two videos, we can see the following parallels:

• Both characters use the Yellow SOUL mechanic.
  
• Both characters include miniature versions of themselves in their attack patterns. These miniatures in turn are capable of firing their own attacks
  
• Both characters use bombs that explode in a Bomberman-style cross pattern; the bombs even use identical sprites.
  
• Both characters feature attacks where the player must shoot at their hearts, which fire attacks back at them in turn.
  
• Both fights feature a free turn for the player: Mettaton EX issues a break time, while Spamton NEO is attacked by his own phone (which damages neither him nor the party).
  

In addition to the EX nods, there are a couple parallels to Mettaton NEO in the fight that aren't depicted in the video:

• The flavor text "the stage lights are shattered" in Spamton NEO's fight nods back to the text "stage lights are blaring" in Mettaton NEO's fight (as seen in the image further up).
  

• Spamton NEO's "Check" description reads "He's his own worst invention." Mettaton NEO's description, meanwhile, states that he's "Dr. Alphys's greatest invention.
  

• Spamton NEO's battle theme, "BIG SHOT", samples Mettaton NEO's battle theme, "Power of 'NEO'":
  

• In the Weird Route, Spamton NEO's ultimate attack contains mock-VCR HUD icons that read "WRECK" and "SPEW," which aren't present when fighting him on the normal route. Footage of the fight can be seen below; the ultimate attack occurs around the 4:24 mark. In the video of the normal route fight further up the thread, his ultimate attack begins around the 8 minute mark. These HUD icons nod back to an attack by Mettaton EX in Undertale, where he can record an attack and rewind it late into his fight, complete with VCR HUD icons of his own. In the Mettaton EX video above, the record/rewind attack first shows up around 8:08.
  

Other parallels:
In addition to the above, there are a few other parallels between Spamton and Mettaton:

• The purchasable Spamton-shaped mannequin in Cyber City wears the same dress that Mettaton does during his parody of Final Fantasy VI's opera scene.
  

• As illustrated in the videos at the top of the thread, both characters speak in all-caps and with idiosyncratic speech patterns: Mettaton's text displays in staggered chunks, while Spamton makes typos and inserts bracketed, mock-advertisement text. The Mettaton fight video also showcases his pre-battle dialogue making a statement that seems to comply with the protagonist's apparent goals, only to immediately respond with "NOT!!!", formatted exactly that way both times. Spamton NEO does this when the player first reaches his battle on the Weird Route as well, as seen in the video of that fight above.
  

Given the myriad of other nods back to Undertale in Deltarune, it's likely that the Mettaton/Spamton parallels are intentional, especially given how thorough they are in total. This probably isn't a complete list of parallels between the two characters, but it does get into how deep the thematic connections between them are.