MapleStory/Monsters

There are many monsters in the world of MapleStory, the weakest of which reside by towns. The most dangerous monsters live deep in dungeons.

Monsters typically roam around aimlessly, even if you bump into them (and take damage as a result). However, if you attack or cast a debuff on an enemy, you will "aggro" it - that is, you will get its attention and it will try to attack you. Most enemies attack by trying to run into you over and over again. Against these enemies, the easiest thing to do is to attack them repeatedly so that they cannot move (your attacks usually have enough knockback to keep them in place). Sometimes it is necessary to jump over them to dodge their attack.

Some higher level enemies and most bosses have ranged abilities that include attacks and debuffs that can slow you, disable you from attacking or using items and abilities, etc.

Note that some monsters float, this means that they cannot be struck by low attacks (such as attacks while ducking).

Maple Island is one of the starting areas (like Ereve, Edelstein, Rien, and others) that only has a few different types of monsters that are all low level and weak. Higher level monsters live in Victoria Island and regions beyond that. For example, Victoria Island mainly has monsters tailored for first to early third job levels (10-70), and Ossyria has monsters level 70 and higher.
 * Monsters in different regions

Items, leftovers, and Mesos
After killing a monster, items (Equip, Use, and Etc. type items) or Mesos may be dropped. However, if a monster drops an item, it should be considered rare. The value of such a "rare" item depends entirely upon the level, if it has a hidden potential, and the item itself (some items are more desired than others based on their stats). Some dropped items can be worth a lot.

The amount of Mesos dropped by a monster is usually proportionate to its level (e.g. weak monsters drop few mesos). When picking up Mesos, there is actually a range of that a given pick up can contain (e.g. if you pick up 30 Mesos, the range could be 28-35 Mesos for that monster). In many cases, monster of similar level drop the same amount of Mesos. The range's maximum value is around 1.5x the minimum value. Monster leftovers (i.e. Etc. items) have many varying uses. Some are used for quests, exchange quests where large numbers can be turned in to an NPC for experience, or for forging materials, such as Firewood that is used to make Processed Wood. These items can be sold to other players that need them, or sent to your other characters if you know they will need it.

Crusader Codex
Crusader Codex is essentially a book which every player is given that catalogs monsters. When the Crusader Codex is added to the game, monsters begin dropping "cards" that contain detailed information on them. Each individual monster's card can be collected only once.
 * Availability: see MapleStory/Availability.

Event monsters
Event monsters are temporary monsters that are only in the game during an event. Some events are seasonal and may last much longer than others. Event monsters are usually themed, so, for example, during the winter there are usually snow or holiday related monsters. Some event monsters are carried over from very old events in MapleStory, such as the Cat and Frog during Halloween. These old monsters usually do not give any good bonuses. They may drop some event-related currency or quest items.

Most event monsters will randomly appear on a specific map or region and have a low respawn rate.

Some event monsters can only be reached through event quests available through the lightbulb over your head. This will take you to a zone where you and all other in your channel can combat special event monsters.

Bosses
There are two types of bosses in MapleStory: strong monsters with low respawn rates that appear near the end of a given region's set of maps, and raid bosses. The first type of monster is more common and less dangerous. The second requires a party to defeat unless your character is at a very high level (~100+ depending on the boss) and you have high damage output.