I've had good luck with my OTTB using a combination of beet pulp (twice daily), canola oil, and high quality free-choice hay. Beet pulp and canola are both very affordable.
Dac oil. Fuel. Beet pulp. You need to watch the protein that you are having them intake. Talk it has your vet as well , your vet could help specialize a meal plan
DAC bloom is one of our go-to weight gainers.
Omegatin worked well for my mare...but the best thing I've found is goods hay...and lots of it!!!
my mare has done amazingly with about 1/2 cup of black sunflower seeds daily. high in fat + cheap and lasts forever
Beet pulp helps to an extent
Weight Builder from Farnam
Beet pulp x2 a day with your choice of grain, and Senior Weight Exelerator from Pro Manna. Only way I've kept wight on my 7 year old ottb.
Alfalfa paired with Dumor weight gain worked great for my saddlebred. He's a really hard keeper.