when an 8 yr. Old suddenly drops weight, it is my opinion that it would be a good idea to get a vet out there ASAP.
We can all guess that it's worms, teeth, piggy pasture mates, etc....but the truth is, only a vet can give you the answers you need.
She does look thin to me, but then both my horses are weighty .... one plump, the other just plain fat.
After health issues have been ruled out, you may want to look into her feeding regimen.
One thing I swear by for both my horses is probiotics. I know some will say horses don't need it but we have two horses with two reasons why I think they do.
Our OTTB was emaciated when we got him, his body score was a 2....that on top of the fact he is a TB and they are supposedly hard to keep weight on. We put him on a probiotic supplement to maximize his ability to digest his food and garner nutrients from it.....a side benefit is that he never gets loose bowels from worming.
Our fatty lumpkins draft does not get grain or ANY fortified feed due to her weight, she is strictly on hay with a few hours of grass time a day. All hay is not created equal and varies in it's nutritional value from year to year and cutting to cutting. She take a vitamin supplement from Smart Pak for easy keeper fat horses, and it includes a probiotic to help her digest the vitamins in her supplement and her hay.
Once health issues are looked into and hopefully ruled out, turn your attention to the diet...and think about adding a probiotic