Mystery squash

I thought that once my mystery squash started to produce, I could tell what kind it was. Initially I believed that this plant was a pumpkin, but now that the fruit of the plant is starting to grow, I'm baffled. This is about the size of a childs fist, light green and elongated. I didn't grow nor eat any of these last year however the seeds came from my compost pile and were volunteers. The plants themselves are quite large and leafy, some have dark green leaves with light green veins/markings and another one is just a medium green with no markings - they are all producing the same fruit though.