If so, where do they grow and what do they look like?
Do wild Pumpkins exist?
Cucurbita spp. are native to the warmer parts of north and south america. Most squashes we call "pumpkin" are C. moschata, which are native to central America, or C. pepo, which has native versions in the southeastern US, southwest US and Mexico.
There are numerous members of the Cucurbita genus that grow wild but most don't produce anything like the vegetables we cultivate. Seminole pumpkin is a ancient cultivated variety which grows wild in Florida to the point where most people think it is native to the area, and produces fruits that look and taste like garden pumpkins.
In addition to some C moschata which I think of as being butternuts mostly there are C. pepo which you mentioned. Varieties like Connecticut Field, Howden, Spookie, Jack O'lantern and Sugar pie. C. maxima is another species where your very largest come from Big Max, Atlantic Dill, or Cinderella( Rouge Vif D'Etampes).
This message was edited Oct 19, 2013 8:42 AM
Here is decent article on squash history. The cushaw group was quite popular in southern climates. Today it is listed as C. mixta, but the older name is Cucurbita argyrosperma. It has wild relatives in Mexico and central america. C. ficifolia is relatively unknown in North America but is semi wild in Mexico. C. pepo, C. maxima and C. moschata have already been discussed. http://www.hort.purdue.edu/newcrop/1492/cucurbits.html
We have Buffalo gourds, Cucurbita foetidissima, which are a common wild native here. They are a herbaceous Perennial (to Zone 5 at least). I doubt they would hybridize with the annuals (too different) and they aren't edible.