Question
I have a large squash plant that appeared in my topsoil/compost pile. It starts as a squash blossom, then a small, elongated, 3" yellowish, then a pale, pale green, oval shaped squash. The taste is perfect at 4". If cooked and eaten any larger, the flesh is a bit spongy. I live in Vacaville, California (zone 9). Our summer weather temperatures are between 85 and 100 degrees (current).

Answer
Sounds like you have a summer squash of some kind. Smooth, elongated, pale green with no striping would likely be a Cucuzza squash. They often curve a bit and can get much longer.

Somewhat wrinkly and pear or avocado shaped, again pale green, would be a chayote squash, which I believe grow wild in California.

Oval or round and not growing larger than 3 or 4 inches and yellowish would likely be a Goldball. They retain a zucchini look to the top of the fruit.

However, summer squashes can cross polinate and you could have some strange new hybrid. Carefully polinate one flower with another and save the seeds and you may get the same thing next year (and then again you may not).