The ultrasound today shows the ovary back to a within-normal-range size and the conclusion is that it was, indeed, a corpus luteum, although the largest one my own vet and two equine reproductive specialists he consulted with had ever seen.
But no need for surgery. And her symptoms of discomfort have disappeared. I think her urine volume is still a bit low but that may be the result of her emptying her bladder when it wasn't really FULL for the last four months so it may need to just stretch a bit. It may have lost a little elasticity in the meantime so I'm not overly concerned about that.
Anyway, it's a relief. While I was willing to do the spay, the last horse I sent to surgery never came home and there's always a risk of something going amiss in any surgery.
And that's the latest from the Ranch.