What great pumpkins you grew! I love when volunteer stuff produces. ❤
A word of caution though so you're not disappointed - because that appears to be a pumpkin/squash from the cucurbita pepo family, unless you hand pollinated it it's very possible your seeds won't replicate the same plant.
If you or any of your neighbours (within a 1/2 kilometer or roughly 1/3 mile) grew acorn, crookedneck, pattypan or zucchini, at least some of your seeds will be hybrid. This is because of it's pollination habits via insects and that it has distinct male and female flowers (unlike, say a tomato or bean that has a perfect flower and is self-fertile).
I would still try growing them, but plant way more than you think you should. Keep only the plants that look like they're growing true to type and hand pollinate the ones you want to keep for future seed.
I regularly bake then freeze pumpkin flesh to use through the winter. So nutritious and delicious!
As for where it came from, if your yard is anything like mine, it was the squirrels! If they buried it deep, it can take a long time for them to come up. I had cucumber plants popping up early August this past year!