Monsters

Monsters are creatures that are fought throughout the Golden Sun series. They appear all across Weyard and make up a majority of the opponents fought in the series, including several bosses. They appear in large varieties and vary in strengths, weaknesses, and abilities. Monsters that are identical in appearance but different in abilities, color schemes, and occasionally size are said to be part of the same "Enemy line". As of Dark Dawn, monsters of the same enemy line aren't necessarily 100% identical but may possess subtle changes such as slightly different weapon designs.

Gameplay Role
In gameplay, Monsters can be fought to gain experience and coins. Most wild monsters (those that are fought in random battles) also drop items which can vary from cheap, common items, to rare artifacts.

In terms of capabilities, nearly all Monsters are capable of attacking physically. Most also possess Enemy Abilities which, unlike Psynergy, don't consume PP and can therefore be used unlimited amount of times in battle. Some monsters, usually but not always stronger ones, also possess limited Psynergy abilities in addition to or instead of Enemy Abilities.

Effectiveness
All monsters have their own Elemental strength and weaknesses. If a monster is felled by a Djinni of the Element that said monster is weakest (lowest resistance), you will have a higher chance at goodies, and get bonus EXP and coins. When gauging monsters, you can use your damaging Psynergies to find out the weaknesses and strengths and then, look at the punctuation. A period "." implies the monster's highest resistance, a single exclamation point "!" means that those are the moderate resistance ratings and three exclamation points "!!!" is the monster's weakness. When felling a monster with a Djinni, you will know that you used the correct element of Djinn when the monster "cries" twice and flashes. The first cry will always be the "small monster" whine, the second will be the monster's normal cry.

Relation to animals
In Kibombo, a girl commenting on some meat roasting nearby, makes the distinction between Monsters and regular animals, indicating that there is an in-universe difference between the two. Various other NPCs hint that this distinction is due to animals coming into contact with Psynergy Stones, spread across Weyard following the eruption of Mt. Aleph.

Trivia

 * Monsters apparently existed in Weyard even before the eruption of Mt. Aleph, as several of them are battled in the game's prologue and within Sol Sanctum. A simple explanation for this is that they lived in close proximity to the mountain and the large Psynergy stone at the Vale plaza.
 * Most enemy lines in The Lost Age contain an extra, unused variant that is a pallet-swap of its cousin variants. Also hidden within the game's code are two completely unfeatured monsters: one that resembles a Nue and the other that resembles a cycloptic crustacean.