They could be a special "slave" or "mercenary" unit trainable from the Market. There could be a global pool of slaves/mercs available to all players. The more players buy, the more expensive they get. Deleting your units (to free up pop) doesn't kill them, but adds them to the global slave/merc pool (you get a small amount of their non-food costs back). Likewise, 10% of all kills adds units to the global pool.