SymmyS was invented by Thomas Mills Hinkle and named by Aren Emigh. The poetic form is a refinement of symmetrical techniques occasionally found in golden age poets. More specifically, the idea of the symmetrical line was first inspired by an observation in a remarkable book of criticism by Dámaso Alonso on Luis de Góngora's great baroque work, El Polifemo.
See http://tommyrot.arrr.net for the symmys home page and a detailed explanation of SymmyS rules and examples.