My guess is a hawthorn.

Unlikely to be hawthorn if the fruits are that large already.
My guess would be that it's one of the native crabs - Malus coronaria, M.angustifolia, or M.ioensis. They can be quite thorny.

Did you eat any?

Nope. I try not to eat anythign I can't indentify. Thanks for the ideas of what it might be. I'll try to get a better look this time around.

I can't think of anything with thorns like some of the hawthorns I've encountered. I probably haven't seen all the native crabs, but I certainly haven't seen any with 2" thorns. I had to fight through a hawthorn thicket while doing soils work in Maine ~20 years ago and still remember wondering if I'd escape with my blood supply and manhood intact.

Thorns on some of the native crabs are easily 3-4 inches long, but they're often more like a thorny fruiting spur, moreso than the pure thorns of hawthorn.
I guess the thing that still makes hawthorn not a viable consideration, in in this case, is forager's description of the fruits being half the size of a tennis ball - in mid-May. I've never seen a hawthorn with fruit half the size of a pingpong ball when fully ripe, much less half the size of a tennis ball early in the season.
Other possibilities would include an 'escapee' seedling pear or quince - seedlings of both species would be thorny.

I should calrify a bit. I stumbled upon the tree in Jul of last year, not May. The tree had also been submerged for more than a week due to heavy flooding and hence had no leaves. Lucky's decription of the thorns is accurate. They weren't really sharp like I'd expect thorns to be. I will make it one of my goals to take some pictures this June and July and maybe that will help. I looked up Malus coronaria, M.angustifolia, and M.ioensis but none of them quite fit the bill.

Wow! By random chance I stumbled upon a name with an image while searching the web, and behold, it was the tree I was looking for. It was a Prairie Crabapple (Malus ioensis). Strangely this tree seems to be more of a central US tree. I'm not sure how it found it's way to Maryland, but I'm positive this is it. Even the flattened green fruit seems to be as I recall.

http://www.phyplt.ipfw.edu/GROUNDS/NativeTreesOfIndianaRiverWalk/PrairieCrabappleIconGallery.htm

Here is the link for easier viewing.

Here is a link that might be useful: Prairie Crabapple

Unbelievable that someone could not find information on crab apple trees with thorns! They are very common and are just another variety of regular crab apple trees. They are not Hawthorne trees, which have a thorny fruit, but are sometimes called Hawthorne apples.

Never heard of thorny hawthorne fruits. You don't mean thornapple, do you?

I've also never of a Hawthorn apple. Hawthorn fruit are called Haws. Hawthorne wrote books.

This is fruit of Sweet Crabapple or Malus coronaria. Some fruit is still on the tree and hard as a rock. The ones on the ground have a nice perfume like a quince. I tasted them and they are interesting but not sweet. Does this fruit have any culinary use?

Any Malus can be made into jelly. Flavour will vary but all will be edible. Include some less ripe ones for a better set.

All you have to do is Google "apple tree w thorns". I've known about then forever only because I read a lot. We planted one in our back yard and it grew quickly, gave lovely shade, and the the little crabapples (any apple less than 2" in diameter) were adorable. Birds loved them!

Graft it over to a more edible apple If it works then you know it's a malus and not thorn apple/ hawthorn.

Let's be careful about common names - a 'thornapple' is a common name for Datura, aka Jimson weed, which is toxic to eat.

Hawthorn fruit - called haws or berries - are edible although not always very tasty. And hawthorns may or may not have thorns...it depends on the species ;-) The fruit does not have thorns and looks more like a rose hip than anything else (also closely related to a rose). When ripe, hawthorn fruit will be a dark orange to red, not green (can be a golden green color in the not common Mexican species). And usually are quite small in size.....seldom over 1/2" in diameter.

Crabapples are completely edible as well but again, not always very tasty, especially when raw. But they do make great jellies and are good addition to ciders, as well as being a helpful pollinator for apples. These also may or may not have thorns.

Modern day apples are hybrids and have no thorns....ever :-)

Although most kinds are red and below 1 in. in diameter hawthorn fruits also come in black, purple and even "blue". The wild hawthorns native around here are black for instance.

Among horticulturally important kinds the fruits of C. crus-gallii are 1/3-7/8 in. wide, dull red, those of C. x lavallei are 3/4 in. long, orange-red, C. mollis scarlet red, to 1 1/4 in. in diameter, and C. phaenopyrum 1/4-3/8 in. wide.

Did anyone see the recent CBS Sunday Morning? HERE is youtube of same. Check out 2:46. Very similar to the native American Malus coronaria in my pic. It goes on to say these apples were used exclusively for cider. I also remember a story about Thos Jefferson using them for one of his cider blends.

sounds like flowering quince to me, late summer I start seeing the fruit described on my flowering quince bushes (many stems) (many thorns)

Hawthorne's that I have produce fruit more marble size, and crap apples not much bigger. So, I would say its flowering quince, Chaenomeles speciosa.

Here is a link: https://en.wikipedia.org/wiki/Chaenomeles

I have a volunteer apple tree that has produced about 6 apples in the past 10 years. I believe the tree came from an apple core one of my kids ate and threw in a small open space next to my driveway (the few apples we've gotten are the size, shape, color, and flavor as a golden delicious apple) This tree has spiky spurs on the small side branch ends -- but they are like iron -- once I stepped on a pruned branch that went through my sneaker and punctured my foot. Two years ago, I removed all the small branches and drill-grafted scions from my other 3 dwarf apple trees (fuji, pinklady, and cox's orange pippen). 20 grafts out of 24 have taken and are vigorous (any new shoot on the original trunk or branches gets rubbed off because of the nasty spikes they would develop -- not to mention that stock was not producing) .