Can anyone tell me what kind of squash this is? Last year we planted some hybrid yellow round zuchini. We had one going bad and my husband pulled it and just threw it in the garden. This year we had some volunteer plants from the seeds. I understand that for heirloom seeds you can expect the same variety. The same is not true from hybrid seeds as you can not know what you will get.

What these plants are producing is unrecognizable. The squash are 6 to 8 inches long. The shape is identical to a yellow crookneck. However, the skin is green with lighter green or white stripes.

I decided to cut in and see what I found. The seeds and skin leave no doubt that it is some type of winter squash. I did bake some as I would winter squash but the flavor wasn't recognizable and was quite mediocre.

I have searched high and low on the Internet for different types of squash and haven't found anything close. Does anyone know what this may be or what it could be used for?
To me, it looks a bit like a birdhouse gourd, albeit a different variegation.  Or perhaps a kabocha squash, which to me has a rather mild and mediocre taste and texture.

I guess you've ended up with some wacky gourd hybrid, resulting from open pollination of your desirable (F1?) hybrid. As is the nature of hybrids, the result could be caused by pollination either with itself, or something else in the area.  Either possibility could be responsible for the Frankenstein's monster you produced.  If you started with a zucchini, you've probably got some Cucurbita (e.g., vegetable marrow as @NBenatar suggests).  

Have a look at other links from Wikipedia's list of gourds and squashes, or one of a number of gourd identification charts such as this one that will help you narrow it down based on the nature of the inside of the fruit, leaves, etc.

On the bright side, you may have a squash on your hands that has never existed before!  On the other hand, it may not be worth much at the market. :)

"It's alive!!!"