Have you recently fecal tested her? If yes and she doesn't need to be dewormed, rice brain is a great option for horses that need some more fat in their diet without a lot of energy/calories. Same goes for supplements like Cool Calories and Fat Cat. I would also recommend taking her off oats and Omolene because those are both very high energy feeds and switching to a starch and sugar controlled pelleted feed like Nutrena's SafeChoice and rice bran to supplement it. And of course, make sure she's getting plenty of hay and forage. I'd recommend a stemmy grass hay for plenty of chew time.
Oats, in my experience, tend to give the horse unnecessary “stupid” energy. Maybe try a grain without oats, many brands also have specific feeds for fattening up that don’t give extra energy. Most brands have websites that offer descriptions on their feeds, or atleast have descriptions on the grainbag. I feed my horse who gets really affected by oats marstall haferfrei (oatfree), it’s not branded as a fattening feed but it’s a good basic müsli, which is what you are going to need in the long run once you get her to the desired weight. And as Scott Leigh said, make sure she gets enough hay/grass! Hope this helps :)
I give her Omolene and Oats
What types of grain do you give to her? Does she have access to enough hay and grass (which is the most important part of their feeding)?
For my horse I use 'Destrier Longlife' supplement, it's great because it helps him to get back in shape while not giving him so much energy 🙂
I’m concerned becaue she’s getting skinnier as time goes by, but we can’t give her grain or any other food suplement because she immediately starts bucking and rearing really hard.
I have a 7 year old energetic chestnut mare.
Before I bought her she was fluffy, she ate 7kg of grain which was really bad cause she was all crazy and bucked and reared all the time for no apparent reason.
So when I bought her we stop giving her grain and she became more docile and things have been working really great. She is a jumper (90cm-1meter), works 6 days a week between 30-1 hr.