Could wild birds have dropped the seeds and they grew? I have netting over my duck pen but wild bird poop could still get in if they were doing a flyover. lolNo way! - I have read about cross-pollination between different members of the Cucurbitaceae family, but that would impact the offspring (seeds) of that crossing, not the fruit of the current generation.
And those are genuine Butternut squash.
And i have no idea where they came from.