The name Dennis is an English baby name. In English the meaning of the name Dennis is: Dionysius is the mythological Greek god of wine responsible for growth of the vines and the originator of winemaking: (equivalent to the Roman god Bacchus.).
Fibromyalgia is a chronic condition that causes pain, stiffness, and tenderness of muscles, tendons, and joints. It is also characterized by restless sleep, awakening feeling tired, chronic fatigue, anxiety, depression, and disturbances in bowel function.
I suspect, however, that this parallel is fibromyalgia. This condition occurs in some susceptible horses when there is a huge build up of lactic acid in their muscles which causes extremely severe muscle damage, massive amounts of pain and distress and in severe acute cases the horse can die from it.