I'm glad you decided to help out and foster her. I would find it hard to believe that she would be pregnant, I'd be more concerned about possible worms causing a big belly, unless you know for sure that a stallion had access to her at one point or another. I also believe that euthanasia should not be overlooked depending on the situation, but I would always ask a vets opinion first, unless it was obvious that it would be the better choice, i.e. colic, or something similar. I had a 3 year old rescue, a cute grey and white arabian paint cross who's owner moved, and abandoned his two horses. I had worked with the horse for a year before the owner left, and for about 6 months after, but in the end had to put him down. It was hard and sad to have to deal with all that, as I had tried so hard not to get attached, since the owner had said he was coming back, but I couldn't help it lol. At least I can be happy knowing that I gave him the best possible year and a half of his short life, and that he was well loved. Even with that outcome, it was one of the most rewarding things I've done.