First of all, before any athlete goes into training, they are evaluated on their current diet and exercise and abilities. Then their diet will change with the amount of exercise they will be doing. If your horse was on an every day type feed such as a basic sweet feed and two flakes of hay per day, her feed should have been changed to one with more protein and more quantity of hay during her training period to sustain her weight. If she was a little overweight going into the program, she still would look like she lost weight because the fat would turn to muscle, giving a more lean look to her physique. Also, at 12, she is beginning to get to her middle age. Her body is changing and she may not be able to move like she did at three and four. Things like arthritis and joint pains can start. I don't think she needs a senior feed at this time, because you are keeping her active and as for a weight gain supplement, not at this time. If you're going to keep her at a lower activity then when she was in training, she should gain weight with just a little extra feed and hay if there are no other health issues. If you want to be able to do the riding that you have just had her trained for, you should also be fit. She needs a feed that will give her the vitamins, proteins and nutrients she needs to keep moving at the amount of exercise she is required. The same is for the rider. You are the other half of the team. One must think of a diet as a way of keeping your body in the best running condition for what it is to perform. The higher the performance, the higher grade of fuel intake it needs. In other words, treat her and your bodies like high performance race cars when you are in an high excercise routine. Also, if she hasn't done much and now you are working her more, be careful of her aches and pains. If you ache, she might too.