What am I growing
We cannot identify the squash in the photos. If you saved seed from last year, most likely the volunteer in your garden is probably the result of cross-pollination between your squash with another member of the same species. The plant that crossed with your squash may even have been growing in a neighbor's garden.
Also, this accounts for some of the bizarre looking squash-like fruits that grow on plants emanating from compost piles. Take a look at cross pollination and fruits not true to type on our website http://extension.umd.edu/hgic/fruits-not-true-type-vegetables