What kind of squash?
I planted butternut & spaghetti squash this year. Spaghetti from packaged seeds & butternut from saved seed. Both are fruiting, but I have also noticed a squash that appears to be coming on the spaghetti squash vines that has a dark green skin with striping that resembles a zucchini but the shape is more like the unripened spaghetti squash. What could have hitchhiked in on my saved seed?
First thing, is it possible you got one of your saved seeds into the spaghetti squash group? Saved seeds often produce plants that are unlike the parent, because the parent was a hybrid plant. However, squashes are all very closely related, so even purchased seeds may produce some unexpected progeny. This article explains more about hybrids and their seeds: https://www.gardeningknowhow.com/garden-how-to/seeds/non-hybrid-seeds-vs-hybrid-seeds.htm