I used butternut squash because a butternut squash was the oldest squash I had hanging around my kitchen. I bought a couple of different kinds of squashes last time they were dirt cheap at my neighborhood's Mexican fruit market, which is also where I got the chilis. Big bags of them, so I'll be eating this stew or variations thereof all winter, I think.
Kabocha or pumpkin would work, too.
There are lots of variations on pork/squash/chili stews. They are Mexican/Southwestern rather than Spanish. They all use one or another type of Mexican chilis. And, chilis and squash are both "new world" foods that are indigenous to the Americas.
Some recipes use dried anchos, some use fresh poblanos. There are also quite a few variations that use pumpkin.
The recipe that I started with came from Epicurious/Bon Appetit but I tweaked it quite a bit.