If you Google "muscle waste on horses", you will get a lot of hits.
Muscle waste is associated with various types of equine diseases and is not to be confused with "no muscle tone" because the horse doesn't get enough excercise.
My 24 yr old has muscle waste from a metabolic disorder he has. He is on a special diet.
Some other diseases that cause muscle waste are EPM, EPSM, cushings; all of which require the horse to be on a special diet geared toward each disease.
Again, if you Google muscle waste, you will get a lot of credible hits to educate yourself