My new horse is a bit underweight so any tips on helping her gain more weight?

Renew Gold and Alfalfa Pellets might get costly over time, but work great on gaining weight

You want a high protein feed or hay -

Be sure she always has grass to eat when she wants, add cereals and if she still doesn't put weight on call a vet 😁

No health issues.. Previous owners just weren't taking very good care of her