My one mare lacks muscle even though I ride her all the time, what's a good muscle builder supplement I can add to her feed??

More specifics with your question could be helpful.

You "ride all the time" but what does that entail? 20 minutes of long trotting with the back hollow? Or 20 minutes of asking for collected loping circles, hind end engagement, and softness at the poll? Even those two scenarios are going to be a big difference in muscle development.

Just like as a human, if you run on the treadmill for 60 minutes versus lifting weights for 60 minutes. One is going to build more muscle - the other one is going to build more cardivascular.

What's your horse's current living situation? Turned out on pasture? In a stall? Small run?  How they live their life could also make a difference in their fitness level, or lack thereof.

Also, what are you currently feeding your horse right now? Hay? Grain? Current supplements?

Amino 15x by MVP . Forco usually helps as well, even though it's a prebiotic. 

Amino Acids. I had some good luck with purina supersport in the past. 

MVP Amino 15x. I've seen great results using it on anything from yearlings to my semi-retired 19yo.