Volunteer

We had some volunteer tomatoes and squash come up in our garden this year.  For the most part, we let them live to see what they would become.  We ended up with four different types of cherry tomatoes and a whole lot of delicata squash.

One volunteer came up outside of the garden, however, and this one we are still curious about.  It's about the size of a watermelon now, but it's definitely more of a squash-type thing.  I'm inclined to let it grow til the plant dies to see what happens, and Millie is doing her darndest to protect it from the influx of mice scurrying around.

Anyone have any idea?