For those who have not completed games in the Xenoblade Chronicles series, this page contains spoilers regarding the plot. Discretion is advised.
Unique Monsters (Japanese: ユニークモンスター, yunīku monsutā) are enemies in Xenoblade Chronicles that have higher stats and potentially unique skills — similar to minibosses or superbosses in other games. Unique Monsters have unique names and there is only one of each of these monsters in the world.
There are 157 Unique Monsters, the only ones giving Affinity Coins when defeated. Five of them are superbosses, and eight of them are quest exclusive. When a Unique Monster is slain for the first time in a given playthrough, each character of the entire party will earn an Affinity Coin. Slaying the same monster again will not give additional Affinity Coins. However, in New Game +, the Affinity Coin is carried over, and one can slay the same Unique Monster in the New Game + playthrough for an additional Coin. The team can then earn a total of 157 Affinity Coins per playthrough by defeating every Unique Monster.
All Unique Monsters have a special frame when looking at them. Story event bosses will also have their own special frame, but are not considered Unique Monsters since they do not drop Affinity Coins.
If the party slays a Unique Monster that is not quest-exclusive, there is a chance for it to respawn upon revisiting the area. All normal enemies can be summoned on demand by saving the game in their vicinity and then reloading the game, or by skip traveling to any landmark (even if the party is already standing at a landmark and skip travels to that same one), but for Unique Monsters, it is not guaranteed that they will respawn every time. However, the actions of save+reload or skip travel can still cause a Unique Monster to respawn alongside all the others in the area when the party comes back, so persistence is key.
Unlike other quest objectives in the game, such as those that require items that spawn in Red Item Orbs, where you can complete the task first and then accept the quest afterwards (most commonly with the Search Quests), if the party defeats a Unique Monster before accepting a quest, that quest objective will not be considered complete and, as such, the party will have to search for the Unique Monster and defeat it again, likely using the actions noted above until it respawns.
Unique Monsters are usually sight enemies, although there are exceptions, particularly when the Unique Monster belongs to a sound-based species, such Vangs and Arachnos. If a Unique Monster has a detection type (such as sight, sound or ether), it will always attack the party when it sees, hears or detects an ether attack, no matter how much higher leveled the party leader is than the Unique Monster. Some only appear at certain times or during certain weather. For example, the Armoured Rockwell appears only in rain. Some Unique Monsters spawn only after accepting a certain quest (quest-exclusive Unique Monsters), and some Unique Monsters will only spawn after progressing to a certain point in the plot such as the superbosses. A few Unique Monsters, such as Indomitable Daulton and Veteran Yozel in Satorl Marsh, will spawn alongside other members of their species and appear to lead them in a group. These partner monsters will generally always be of the Kindred Enemy type, and will defend the Unique Monster in battle, even if one attempts to lure them away.
A specific track, "You Will Know Our Names" (also known as "Those Who Bear Their Names"), is associated with battles against a Unique Monster (except against Frenzied Bana), a track which references the Unique Monsters' tendency to literally 'bear their names' in their tag.
Some Unique Monsters are missable, particularly the eight quest exclusive ones. If an area becomes inaccessible at any point in the game, so too do all of the monsters that were there. Defeating a Unique monster gives more Experience Points and Art Points than minor enemies at the same level.
Advanced Books can be dropped by Unique Monsters with level 45 and more, except the 11 Unique Monsters in Sword Valley and Galahad Fortress as well as the 3 quest-exclusive enemies Conflagrant Raxael, Unreliable Rezno and Brutal Gravar.