In response to your request here is the start of my vision of new mobs for a bestiary!
Note: These are just my fun ideas and would love to see an iteration of them in the game! Feel free to add any that you want or comment on the ones you like!
The only rule is you have to use the standard paint program.
Doubloon Master is a semi-rare mob that spawns in coastal lost areas that are pirate focused. Has a flintlock pistol in his left hand and a hook in place of his right. The eye patch is a must! He is initially aggressive but once his health lowers to half he will attempt to run away and drop snares behind him. Every time he hits you with a ranged attack you lose 5 gold, but fear not for once he has been killed you get a “Doubloons” worth of gold back (and your original lost gold if any). Yes I misspelled Doubloon in the original 
Enemy Type: Lost
Primary attack: Ranged
Special ability: Forces players to lose five gold per ranged attack
Starmetal Golem is a mob type that has a true mining value! Spawning in all territories this golem is very rocky and very angry. His most notable features are the starmetal veins running all over him, his overly large sized starmetal hand (Only one!) and his one eye that is a starmetal rock. You will have to be able to dodge or block effectively to destroy one of these guys but if you can get past his high health and armor a mining treasure awaits.
Enemy Type: Nature
Primary Attack: Melee with focus on AoE stuns
Special Ability: If hit with direct attack from his big hand the player is knocked back 40m.
Special Characteristic: Once he is killed his body becomes mineable and the player will receive 200 starmetal ore. Add a rare chance in Shattered Mountain to spawn an Orichalcum golem.
Note: Good opportunity to add the starmetal meteors with this guy as he could spawn with them!
Bats are as exactly what they sound like. Nothing fancy, no corruption, just very evil night time only spawning bats. First flying creature in the game that will pester players randomly in the night. While not strong individually, they can be very deadly in large groups.
Enemy Type: Nature
Primary Attack: Melee
Special Ability: If “bitten” by a special attack you will bleed and attract other bats until all are killed.
Swamp Leeches are nasty little buggers that spawn in shallow water only in Reekwater and Brightwood. Once bitten they cause rend/slowing and are a true nuisance. Not everything is a loss when it comes to be attacked by a leech however as on rare occasions when killed a leech will drop craft mods of various animal types it has devoured!
Enemy Type: Nature
Primary Attack: Melee/sucking
Special Ability: If hit with “Prey Latch” player is slowed by 50% and Rend is applied
Special Characteristic: On rare occasions on a kill a leech can drop various craft mods
Withered Vara Knights are the definition of a brute. Strong, aggressive, and a lot of armor to back them up they will charge headfirst into your group. Having landed on the beaches and strayed from the main force these knights have slowly starved into empty husks of their former selves. Be careful of their mighty swords for they have not forgotten how to use them!
Enemy Type: Lost
Primary Attack: Melee
Special Ability: This enemy has two specials. The first being “Armor of the Undead” which after striking the knight three times a player is given poison for 5s. Second ability is “Shriek” which activates when the knight is at 50% health. This ability causes knockback and summons 4 withered mobs to assist the knight.
Korum Tadpoles love fishing hotspots! Doesn’t matter if the water is deep or shallow the Korum Tadpole has adapted to survive in both enviroments. In the shallow water they wait in stillness for bullfrogs and fish to swim near the surface, in the deep they deceive their prey by acting like a sunken log and wait for them to come for shelter. Players in the world have a chance to be snagged by this beast and if hooked the Korum Tadpole will swim to the shore very quickly and engage a player. Beware their bite!
Enemy Type: Nature
Primary Attack: Melee
Special Ability: “Big Jaw” is an ability that causes a large amount of physical damage and if the tadpole is able to strike a player with it the tadpole will become enraged and gain 10% damage per “Big Jaw”
Special Characteristic: Drops “Korum” bait that will give a larger chance of catching rare fish for 5min (Tradeable)
Mimic is last on this list but not least by any means! The old fashioned and tested to be true mimic will be in areas all across the world. This fella doesn’t need much explaining at all.
Enemy Type: Corrupted (Due to being a corrupted chest)
Primary Attack: Melee
Special Ability: Hides as a normal loot chest in the world and needs to be defeated once activated to get the original chest. If a player does not respond to an ALERT within 1s the mimic will stun and attack the player for 3s dealing 35% max health as damage.
That is all I have for now. If you enjoyed these please feel free to add some yourself and I will continue as well!
