Some kinda buttercup ?pumpkin?
The skin pattern says vining summer squash, but the shape is pumpkin. So I’d go with the latter.
Looks like a Kabocha, a type of Japanese pumpkin. Or maybe a Buttercup squash, but looks more like Kabocha.
I assume it’s a volunteer? Could be lotsa things, given the way cururbits hybridize.