Repository for Miscellania

Evidence in favour of only Sans being from the Deltarune universe:

Sans appears to bleed while Papyrus doesn't.

Papyrus has (supposedly) never seen a human before the events of Undertale.

Papyrus doesn't (supposedly) recognise the sun.

If Sans ended up leaving his original brother behind, that would explain some of his desperation to return.

Evidence in favour of both Sans and Papyrus being from the Deltarune universe:

QC's/Snowdin Shopkeeper's comment about the brothers just showing up one day COULD just be about them suddenly moving to Snowdin, but either way it does suggest they came together from wherever it is they were before.

The "green grass" comment in the Q&A is intentionally ambiguous, but if it's not a cute reference to the Kickstarter trailer it suggests Papyrus is from somewhere other than the underground.

Less evidence and more a question, but if it really is just Sans hopping dimensions, what the hell happened to the original Sans Undertale? How did Sans Deltarune and Papyrus meet? How much does Papyrus know of what's going on (he clearly has some idea about his brother's secrets)? What would all this even mean?

I feel like Green Pippins even bringing this up, but: the non-unique enemies on the Sword Route seem to correspond to important characters in Undertale (and specifically Undertale due to the inclusion of flower enemies; the cats could be stand-ins for ghosts although that is admittedly a reach). There are, however, no skeletons to fight, which might indicate the skeletons weren't originally a part of Undertale's world.

Miscellaneous oddities:

Papyrus has a vague notion of what a toilet is despite toilets not existing in the underground (but the underground has some access to human media which could presumably mention toilets).

Papyrus is WEIRD. He breaks physics in a different but equally noticeable way as Sans does and exhibits other strange behaviour that can be dismissed as comic relief or just a part of his quirky personality but which could also point to something more. There's also the fact we know absolutely nothing about his past from before he arrived in Snowdin. However, it's impossible to prove this means anything without further evidence.

Sans has seemingly teleported in the Light World in Deltarune the same way he does in Undertale???

More a vibe than anything concrete, but Sans and Papyrus sure feel like they've known each other their entire lives.