[Sans isn't taken aback at first. He assumes this guy is just a monster, like him, and it's not all that weird. Perhaps someone was playing a big joke. Sans could jive with jokes, and eventually the punchline would come on out.

But then the fellow speaks with such familiarity. Sans can't help but do the equivalent of raising his eyebrows.]

Winter's kinda what I'm used to, honestly.

[As to be polite, Sans reaches to shake hands with the furred creature; so politely, he didn't even try any gags!]

But...I think you got me mixed up with someone else, buddy. I don't believe I've met ya.

[Though the amiable nature and excessive size does remind him of someone he knows, this is hardly Asgore.]