Spawning at night on green grass (or grey in ethereal) these mobs wander around looking for a player to eat. Drops 1-5 dirt when killed.
Dungeon Master
Spawning below -70 underground DM's have a tendency to hurl fire balls at unsuspecting players and can cause major damage, but get too close and he will switch to dogfight attack. Can drop mese or diamond when killed.
Lava Flan
Cute as they may look lava flan wallow in their namesake (no, not flans) and get curious about players who wander by, forgetting that they can burn you and cause damage. They have a 1 in 5 chance of dropping lava orb when killed, but if they die in water then pray they dont solidify into an obsidian flan that shoots shards and destroys all around them.
Mese Monster
These mobs are territorial and spawn below -20 and will fire mese shards at passers by, so best avoided. The deeper you go the tougher they seem to get. Will drop mese when killed.
Oerkki
Found in dark areas like most monsters Oerkki wander the caverns stealing away torches on the ground and attacking anyone found in that area. 1 in 3 chance of dropping obsidian.
Sand Monster
The hot deserts are home to these guys who spawn at any time of the day to attack players. They drop 3-5 desert sand when killed.
Spiders
Snowy spiders are found on higher cold areas, spitting Tarantula's in higher jungle, small Cave spider below -20 and Mese spider near areas containing the ore and Crystal spiders only in Ethereal's crystal biomes. Some are docile during the daytime and will drop string when killed.
Stone Monster
Found underground in dark caves these mobs seem to be zombie-like in fashion with a tendency to rush a player in the area. can drop torch, iron or coal when killed.
Tree Monster
Found atop tree's at night time they drop down and look for food in the form of players and animals. Can drop saplings and sometimes an apple or three depending on type. Also note that green tree creepers exist and sometimes go boom.
Land Guard
These huge monsters roam the land in cold, hot and temperate areas and don't like players wandering around their domain.
Fire Spirit
Fire Spirits will not tolerate players roaming around their domain and will fiercely attack until their dying puff of smoke. Will drop it's spirit and some fire dust when using ethereal.