I think it’s some kind of pepo. It looks a little like the tiny ornamental pumpkins I grew maybe 5 years ago, but those were less than 2 inches across. I don’t remember what I planted in this location, but I’m pretty sure this wasn’t it.
The plant right next to it is a very obvious pie pumpkin.
Jack be little?
Could be, I suppose. I’m always intrigued when something I never grew pops up! 
Jack be little pumpkin (picture from Gurneys catalog)
(Chuckle.) If you didn’t plant them, I wonder where they came from? Seeds sown by a previous owner of your land, maybe? (Or of course, they could have been in the Reckless Pepo mix, or a wrong seed in a seed packet you planted, or something.)
Could have been a lot of things. I guess I should alter that statement to say I know this is not what I thought I was planting. I have never deliberately grown a squash that looks like this.
Best way to find out would be to let it mature fully, save the seed, and plant at least 16 of them in isolation next spring. The fruit at season end should give an indication of the ancestry, if you apply the various phenotypes to the Punnett square.
To me it looks like a melon of some kind, maybe cassava or canary melon.
The leaves and the blossoms definitely say squash. I may grow it out, but that depends on whether or not it has traits worth keeping. Otherwise I’m just not interested enough to do the work.
@Lauren Can you guess at the type of cucurbit it is by the leaf shape etc? Maybe that would help, if you knew what family it belongs to?
I find this guide by @Joseph_Lofthouse so clever
Lofthouse how to identify squash
I wish that i would finish that web page Maybe add a couple species.
Such size, shape and densely ribbed fruits are on Baby Pam and Wee-be-little squash varieties, you may want to compare.
