looks like a butternut type. Can used a summer squash before the skin hardens, but mostly considered a winter squash in which case you let it mature on the vine. If it is a butternut, it will ripen to typical butternut tan. There are varieties like the Trombocino/Rampicanti are mostly used for summer squash

I don't recall ever planting butternut, so I wonder if it could be a mongrel kabocha. The second year of a great kabocha crop I had a lot of weird (some inedible) squashes.

Kabocha types are c. maxima. Offhand I don't know of any C maximas with elongated bell shape. Most are close to round, oval, or standard pumpkin shape. They don't cross with butternuts.

They can cross but it is not common and the resulting hybrids look much more like the kabocha than a butternut. Cucurbit phenotypes are close enough for the most part that pretty much any species can cross. And the results can be startling if not necessarily very edible :-)

The squash in the photo looks to me to be an unripe butternut more than anything else. But if you have never planted butternuts (or discarded the seeds in your compost) I have no explanation as to how it came to be growing there.

Yep, looks like a butternut . Maybe an animal or bird deposited seed there from elsewhere or, if you planted it thinking it was something else, the transplants or seeds might have been mislabeled.