Dieting Appy Mare

I finally got my gelding home and I am keeping a mare for my friend to keep him company. I had no clue this mare was so obese. She is a bit over 15 hands and when we brought her in, the weight tape wouldn't fit around her!
I got the two separated so I can feed them individually, and now the mare has dropped about 100 pounds in a week. I know in humans, it is dangerous to lose too much weight too fast. Is this also a concern in horses? I am not starving her, and her owner knows she needs to lose about 400 pounds total.
She is currently getting 1-2 flakes of hay twice a day and a cupful of oats with her supplements at night. There is no forage to speak of in the paddocks. She came from the Willamette Valley to Central Oregon 2 weeks ago, which means she went from 24/7 grazing on rich pasture grass to this situation. (We put her on probiotics for the transition and she has done great).

So, do you think I am making her lose weight too fast?

Bumping (how annoying! I know I know)

She is down to about 1250 according to the tape. I measured her belly and she is 79 inches around. 75 inches from point of chest to "butt cheek" and has 8 inch cannon bones and stands at 15 hands exactly.

Her crest is getting much softer, so I am relieved about that.

I still wonder if there are risks to losing too much weight too fast?

I don't think loosing 100lbs in a week is safe at all. And 1-2 flakes of hay per feeding is not enough, in my opinion.. unless they are some big flakes. Your only feeding 2-4 flakes a day.... I know you want her to loose weight but completely cutting her food out does no good. Especially because she use to graze 24/7. Do you ride her? What breed of horse are we talking here, pictures would be nice

Being that she is 15hh I would say she should weigh around 1000lbs, give or take. So I would feed her as if she were 1000 pounds, so about 15 pounds of food per day. You should weigh the feed, cause one flake could weigh totaly different from the next. And then weigh the oats. However I would not give her oats at all.
I just put my boys on a diet ( they were free feeding on a slow feed round bale) and I was completely shocked how much 3 flakes weighed. I used 2 flakes from one kind of hay and 2 flakes from another and that weighed 8 pounds, I was giving that 2 x a day in slow feeder bags and giving 2 pounds of beet pulp. I was still over feeding. I now use a fish scale and keep it in my pocket.
Hope this helps

She is an appaloosa. I am not riding her, her owner has come out and ridden her once in the 3 weeks she has been here. And in the past 4 years she has only been ridden occasionally, so I don't think it's fair to ask her to work too much when she is this unfit.
According to her owner, she usually only feeds one ton per year (my jaw dropped when I heard that!) and that was valley hay. She is getting orchard grass hay now which is fairly rich. 2 flakes weigh about 10 lbs I figured out today. So she is getting 20 lbs daily.
My vet always advised to feed 1.5% of body weight as a guideline for weight loss. So if she is 1300 lbs that would be 19.5 lbs.

I feel she is getting enough, just wasn't expecting so much weight loss so fast. I did not cut her to that instantly. I had both horses together for the first week so she was getting closer to 4 flakes, since she kept chasing my gelding off of his feed.

It is too dark to get photos now, but I will tomorrow.

Ok that sounds better because I have some hay where it weighs 6lbs a flake and some that seems like it barely weighs a pound. I just had to check. I also don't think its fair to over work her either. When she slowly starts coming down in weight you can gradually introduce that. I would also ditch the oats too. Find her something better if she even needs anything at all. Make sure to have a salt/mineral block out too

Thanks for the suggestions. She does have access to salt/mineral block now, after 4 years without one (another jaw drop for me!).
I have a very different method of horse keeping than her owner does and I don't want to step on toes, but I also wanted to make sure I am keeping her horse healthy.
I will have to pick up a fish scale so I don't have to drag the bathroom scale out each night, lol!

Wonderful news! The piglet, I mean appy, is still losing weight, but at a much slower pace now. I think she really was just gorging herself before!
I finally got a pic of her, too. I wish I had one from when she was first here a month ago. My neighbors called asking when the twins would be born when she first arrived!

She is a bit chubby!

If she goes without hay you are inviting ulcers. Get some small mesh hay nets which make the hay last longer. She doesn't need supplements nor the oats at this time but more hay. Put the horses together and give both a net and hang them well apart. Your gelding will do better with a net also. Remember, with horses, the faster it goes in, the faster it goes through them. The body takes only what it needs from the supplements and often they just make expensive manure. One of the reasons horses get so fat with round bales is because they only move to get water. For two horses, my preference is to get two and set them as far apart as possible. The horses will eat from one then go check out the other, back and forth. Moving thro deep snow keeps them in shape. If you can get a few bales of oat straw, she can eat a flake per day, perhaps divided. No food value but it provides the long fiber the gut needs without putting on weight.