Thank you all. We won't ever know for sure what happened, as a necropsy was not possible due to the heat and state of decomposition by this morning. It's just SO hot, and she was solid black.
I don't know what to think about the tomato--I only have the one plant (which has dipped down and spread, so it looks like 3 plants), with tiny baby green tomatoes on the bottom half. There are a good dozen horse-toxic plants out here, especially trees, but she's lived here for 3 years with no ill effects, so I just don't know.
About 3 days before she died, Victor got to chasing around with her, and she got really exhausted, so I separated them, hosed her down and scraped the excess water off a few times to cool her down, and she appeared fine after a few minutes. That's the only unusual thing I can think of that happed in the days leading up to her death.
She was just lying down in the grass like she was asleep. No sign anywhere on her of having rolled or thrashed around, no dirty places on her body from rolling, and the grass around her was perfectly undisturbed. It honest to gosh looks like she went to sleep and never woke up.
I'm starting to wonder if perhaps she had a congenital heart defect that we never knew about, and all the running from the other day just pushed it past what it could keep up with? She's certainly never had to work much in her whole life, only being ridden at a walk.
I feel like this property is cursed. This is the third equine death I've had here, and we haven't even lived here 3 years yet. The first two were necropsied, though. My old stallion was just, well, old. And he'd lived a hard, hard life before I owned him, so getting into his mid-twenties was probably pretty good for him. And then I lost a 3-year-old colt to a sand-colic impaction...but in that case, it was obvious he was sick, and we tried everything we could. Both of those horses were euthanized to end their suffering. So there was a lot of awareness there on our part.
Just coming home and finding such a young, fit pony dead for no apparent reason...that's another thing altogether. A sickening thing.