This came up unexpectedly and I’m waiting to see what it’ll be. I didn’t plant any squash or pumpkins this year. In fact I’ve never planted either in this garden bed. We affectionately call it our “pumquash” while we wait.
I have it under a greenhouse because we had our first frost warning this weekend, and I’d really like at least one fruit to mature so we can solve the mystery. This is the furthest along.
To me it looks like a zucchini or squash. If it lengthens it could be a zucchini “Magda,” the color seems right. But I really think it’s a particular variety of squash (or a mutant) that is globular, called “cue ball.” I don’t know how to share a picture, so this link will have to do.
https://www.johnnyseeds.com/vegetables/squash/summer-squash/cue-ball-f1-squash-seed-3359.html
The seed catalog I am looking at shows a light green, yellow, and dark green version. The dark green one is sometimes called “Eight Ball zucchini.” Light green is called “Cue Ball” and yellow is called “One Ball.” Curcubits cross pollinate routinely, so it could also be some sort of mutant - congrats on your cross breeding program! :)
I found another pic from Osborne Seed showing the color variations - will be exciting to see what color you’ve got! Sorry I can’t upload photos, it may be my instance or app bugging out.
https://www.osborneseed.com/products/cue-ball-f1-untreated-squash-seeds