Description of Issue
I recently set my horse Spartan Devil as a broodmare, but I am unable to use the Hypothetical Mating because it says “The mare you submitted was not found!”. Her name does pop up when you start typing in the hypothetical mating, but I still receive the error message.
I am having this same issue with my mare, Dawn Treader. It does not matter if I type in her name manually, click it from the drop-down list, or if it is auto-filled when I access it from her Breeding page.
Edited to add: this mare is also a newly retired broodmare.
This is a very common problem with newly retired mares. Mine seem to have sorted themselves out after a while.
I have the same issue with newly retired broodies. I think the script has to catch up 
Bumping this up, as it’s still an issue (and it will say Stallion Not Found if the horse entered stud in the last year).