What is this
I just answered a similar request that was sent. But with more information I would say that one of the squash you had in your garden last year either cross pollinated or reverted back to a parent. If the fruit was picked young and immature it could be probably used like a zucchini. Once the fruit gets larger, the skin hardens then the seed cavity will be developed and it would be more seedy and mealy in texture. At this stage I would probably leave and see what develops. It will be ripe when there is a color change or the skin is tough to fingernail pressure. Then you could cook and eat or use for fall decor.