My 10 year old JRT has an almond shape and size break in her skin right where her haunch meets her side, at the very top, slightly under it, as though someone was about to cut her haunch the way you would remove a chicken thigh. It wasn't there last night, although I did let her out briefly after dark to pee. There is no sign of a bite, nothing jagged, it's clean (and I keep it clean now). I suppose it could have been cut on the ancient wire fence she climbs through day in and day out for years, but that has no sharp places. I suppose she could have been cut many ways, but I'm wondering if skin ever breaks open by itself.

If it happened all of a sudden (overnight) I would bet it's just a cut. Unless there is an abnormality around the cut (swelling, dry cracked skin etc) then I would treat it as a normal cut.

If it if almond shaped and fairly long it may require stitches to pull the skin back together.

Thanks shazamataz.
It is just about the width/length of an almond and I can't decide about stitches or not... vets here are very expensive.

Can you post a photo of it? We might be able to help you make a decision.

I would suggest butterfly stitches but it has been too long and it may need reopening before being closed again (if that makes sense).

Some small wounds can be closed via second intention (ie just healing on its own until it closes), but that requires special care. I would at least go to the vet to ensure they don't think it needs antibiotics - and if they agree that second intention is the way to go.

You could also do a butterfly yourself at home. I have done them dozens of times. My money is on the fence. You said it was ancient, so maybe it could have broken off in a spot due to weathering. Here comes my usual spiel. Use Epsom salts to keep it clean and if doesn't heal up with in 2 days (or at least start to look like it's mending) then you should seek your vet.

Yes, I think it was the fence - there's a tiny scratch from the bottom of it all the way across her belly.
I'm keeping it clean and it's skinning over already. Poor doggy. She's short even by JRT standards.
She still thinks she has to go guard all the borders.
We have bald eagles who dive bomb ducks and she's no bigger than a duck.
We have coyotes.
We have all the other animals her size too, but those 2 are the ones I worry about. And now I have a cat too. But I let them go outside... and worry.