I'd guess it's some sort of mongrel/hybrid, but I'm not sure what squashes the parents might be. Perhaps someone more knowledgeable than me has some idea.

Definitely not a watermelon, though!

Thank you Caleb! I wanted to add that I cut one open and it is definitely not a spaghetti variety. I think it could've been slightly more ripe, but the seeds were like mini pumpkin seeds and even the scent was mildly reminiscent of a pumpkin. I roasted the seeds and it has a very good flavor.

Serendipity can be magical. If the fruit has promising traits, you should save the seeds and try for another crop next year. Clearly it's a vigorous plant, thriving and fruiting w/ minimal care. At least that counts for something.

It's sure pretty, whatever it is. I agree, keep the seeds and grow a few plants next year.

I have one very like this. The seed package said spaghetti squash, the other hills are definitly spaghetti squash, but this one is different. Package came from Seed Savers. I will save seeds, and now am anxious for harvest time!

I have something just like this! I purchased a seed packet and started them, supposedly butternut but it doesn’t look like a butternut squash! Did you eat it?

I just stumbled onto this thread. I have two of this exact same plant - also "volunteers" in a landscaped bed. There are at least 6 large fruit like the one you have pictured. We harvested one early and cut it open - it's definitely a squash. I think it's some sort of winter squash, or a hybrid of squash and pumpkin? We haven't tasted it yet. I see it's been 2 years since the last comment. Did you learn any more?