Too skinny?

I hear Thrive horse feed is supposed to be really good at adding weight. I have also used K finish from Tribute feeds with good results and empower.

From the angle I can not see if she is underweight, but definitely lacking in muscle, severely.

Looks just like a cushings horse at the barn.

In the picture posted the horse is underweight. I would suggest blanketing the horse, having her teeth checked ASAP, having the vet come out and check to see if she's wormy. Without the age I can't really suggest food but I really like the Triple Crown brand (a bit pricy). At the barn I worked at that was what we fed (Triple Crown Complete and then Triple Crown Senior for our old guy) and it was great! Also consider adding bran to their food, and if their teeth are too poor to be able to chew had ask your vet what they would suggest as a substitute! We gave our old man Triple Crown Alfalfa and never had any issues on it (though I know some people are skeptical about feeding alfalfa).

First, Teeth, dewormer, Second, Have you talked to your Vet? Third, what are you feeding her now? Find the best hay you can, there are some great feeds out there, Talk to your feed people. Someone say blankit, it really would not be a bad idea, but make sure you keep it as clean and dry as you can other wise your waisting time blankiting. IF you can't keep it clean and dry your better off leaving it off. Don't waist time talking to us Talk to your VET.

Can you provide a more up to date picture please?

Def. Agree. Get your vet out there! And if you don't have a waterproof blanket, it's not worth the time since if it gets wet and stays on in the cold that in itself can cause problems.

Can't really tell through that thick coat. If anything, she's a little chubby.

As far as her backbone goes, she just needs muscle to fill that in.

She gets riden almost every day.

In your photo you provided that horse is way too skinny!

No amount of Muscling up is going to fix that horses weight.
That horse needs groceries.

How do you even fit a saddle on that horse with that backbone?
I know you said that photo was old, but I think less riding until her weight is up to par.