This immense volunteer squash plant came from my compost - I did not plant it - and I have no idea what kind of squash it is. We didn’t eat or grow any kind of squash like this last year that would have been in the compost. It is a giant plant and very productive. When young, we ate the squash like summer squash - skin and all, but letting some get bigger the skin is now getting harder and the insides drier like a winter squash. What squash is this? I can’t find anything like it on the internet. And how can a parent squash produce offspring different from themselves (Which is what I assume happened in my compost bin)?
This squash is probably the result of cross pollination between the acorn squash and the pumpkins (or any other related squash that you have grown). Both of those crops are of the same species (Cucurbita pepo) and can produce seeds pollinated by a related plant. The effects are seen in the following year when the seeds that have been saved or have fallen into the soil grow and produce an unfamiliar fruit (like yours).