But lots of mares and geldings get on well. It is sooooo sad but horses do have to be horses and these things happen. It might have had nothing to do with the other horse, just one of those freak things.
But she'll always be remembered for that amazing test!
It's notoriously difficult to know when a mare will actually foal (3 weeks either side of due date is perfectly normal). Th foal's lungs don't mature til the very end, so as with any species that is premature it's their air exchange that lets them down, and the foal could have been weak for life. So I guess they just let the go together rather than trying to cut her open in her last minutes.