There are many monsters in the world of MapleStory. The most dangerous monsters live deep in dungeons.
Harmless monsters tend to stay by towns. Maple Island is one of the places that has very few monsters and are mostly very weak. Higher level monsters live in Victoria Island, but more dangerous monsters live closer to the center (Dungeon) of the island. Ossyria mainly has high level monsters as well as four major bosses.
After killing a monster, Mesos or monster leftovers (ETC. items) may be dropped sometimes. However, the monster may also drop something rare (infrequently of course), such as an equipment, which can be worth a lot. The mesos dropped by a monster is varied, and the range varies with the monster (and is usually, but not always, somewhat proportional to the level of the monster, so you will find that in SOME circumstances monsters of same level drops the same range of mesos), the maximum value usually being 1.5x the minimum value.
JapanMS released the Ariant Colosseum, more area bosses, and added the Bot catching monsters. The Turkey Pet and the MapleStory Anime Pet Kino were also added.