Mettaton's design is similar to other characters not belonging to Undertale.
Mettaton's normal form resembles the coin-operated robot Cooker from the Wallace and Gromit short "A Grand Day Out".
His form also bears a resemblance to the Craniac robots of ChalkZone fame.
The shape of Mettaton EX's body is very similar to Klaus Nomi, a well-known German countertenor's outfit.
Mettaton is at least partially inspired by Twitter user nerdbotmk2.[2]
Mettaton NEO resembles Bass's Treble Boost form from the Mega Man series.
Along with Flowey, Mettaton is one of the only characters in the game to have a voice clip, exclaiming "Oh, yes!" when he transforms into Mettaton EX, and "Yeah!" when attacked in his Mettaton EX form.
Stepping on a green tile during Mettaton's colored-tile maze will play the same "Yeah" when Mettaton EX is attacked.
The answers to the question What are robots made out of? in his quiz show have several references.
"Hopes&Dreams" references the track "Hopes and Dreams"
"Snips&Snails" and "Sugar&Spice" are references to the nursery rhyme "What Are Little Boys Made Of?"
Mettaton's box form is identical to the colored tile maze machine found in Snowdin.
During Mettaton's singing performance to the protagonist, the program window is renamed to "UNDERTALE the Musical".
Mettaton EX has responses for "sexy", "foxy", and "tantalizing" in his essay question, but since the X and Z keys can't be inputted, they're not accessible in-game.[citation needed]
Mettaton EX originally saved images of all the essays to the computer's hard drive. This feature was removed for being too buggy.[3]
According to the Undertale Kickstarter, there were originally plans for the protagonist to have a robot husband, which is presumed to be Mettaton.[4] This ended up being cut out of the final version of the game for unknown reasons.[5]
Additional evidence that points to this event is an unused variation of the Start Menu that can be found in the game's files and how Mettaton and Napstablook are suddenly included in the full menu.
In Undertale v1.001, Mettaton's stats for his box form has been changed from "10 ATK 999 DEF" to "ATK 30 DEF 255".
Mettaton talks in morse code, which all translates to 6C704D1E2C95628416D42ED91E3354BD, a MD5 hash which can be translated in a MD5 converter. Doing so gives you a link to an image of Flowey saying "You naive idiot.