"Welp, here we go."

So I had a weird thought of what it'd be like if Undertale and Pokemon merged. I really can't imagine Sans as anything but the Champion. I imagine he'd use a mix of types, mostly ones that his friends might use, as well as a Shedinja to reference his gimmick of having only 1HP and relying on evading damage outright.

For his signature Pokemon though, I chose Mega Gengar, specifically a shiny one. It's super-powerful as is Sans, it's Shadow Tag prevents anyone from escaping the fight, just as Sans traps you in his fight, and overall it bears a fair resemblance to a Gaster Blaster, Sans's iconic weapon.