Bless you for fostering. It is a very rewarding experience.
if you check my profile I have an album on there of Reno the Yorkie I am fostering. He was a puppy mill seizure.
As to what to feed her. What are you feeding your other dogs?
A good diet will soon fill her out. I can't advise on what food as I have no idea what dog food is available in Spain.
Thank you for fostering
