Crystal ponies are strange and lovely. They’re made from a sort of living crystal, which is powered by their emotional state. As long as they are hopeful, they remain in their beautiful crystalline forms. These crystalline forms make them hardy and difficult to harm physically. However, if they are bereft of hope, they lose their crystalline forms and return to normal, fleshy pony form.
 
Unfortunately, given that they are made of living crystal, they are quite delectable to dragons, making most crystal ponies a bit skittish about them. (Spike being the obvious exception).