It could be fun to have for a new civ, a unique unit who spawn with a third of his full health.
For this unit it could be a cavalry or range unit. The training time could be regular and the player will have to choose to use them without healing them.
I believe that it could be really intersting to play with them.
For a historical point of view, it could be for a civ who was in siege and fight with an army of casualties.