A unique monster/enemy is an enemy but it has higher stats, making it harder to kill. It has a special name for itself and they is only one in the game. Unique enemies are usually sight enemies but they are expections. Some appear at certain times, like Armoured Rockwell only appears in rain. Some unique enemies have to be killed in a quest. All unique enemies drop a gold traesure chest.
External Links
http://gbatemp.net/t307208-xenoblade-chronicles-unique-monster-location-full-map