(ba-ra-KO-ah)
Native to the savanna, the Barakoa tribesmen travel in small hunting packs. They are one with the masks on their heads, which cannot be removed. Some use bones as clubs as weapons while others use poisoned blowdarts. Each hunting pack is led by a Barakoana elite, wielding a traditional spear and shield. The Barakoa make traversing the savanna quite dangerous, as they can surround their prey quickly and put up a good chase.

When defeated, the Barakoa will sometimes drop their masks as items that the player can wear. Each mask bestows a different, small buff.

Barakoa can also be found in villages scattered across the savanna plain.

(BA-rah-koh)
Barako is the chief of the Barakoa tribe, residing in a stationary seat upon a throne in Barakoa villages. He draws his power from the sun, calling forth sunstrikes, solar flares, and massive solar beams. He can also create Barakoa followers from masks to defend him during combat. As such, his masked children revere him as a god. Unequipped travelers should keep their distance from his villages, lest they be smitten by his heliomancy.

Those strong enough to face and defeat him, however, are rewarded with his mask: the Sol Visage. This mask allows its wearers to breathe life into masks to create Barakoa followers. Unfortunately, the mask does not bestow heliomancy upon its bearer; this power is innately carried by Barako, and thus dies with him. Perhaps there is another way...?

(FROST-mah)

Frostmaws are dangerous beasts found rarely in snowy areas. Their massive size, impressive speed, and powerful ice breath make them near-impossible opponents. If, on your travels, you encounter one sleeping in your path, it's likely best to take a different route and let it lay. The beast guards a precious ice crystal that can unleash pure winter energy when wielded.

(GRAH-tohl)

These adorable crystal crustaceans live deep underground in caves and ravines. Sometimes called “living ore,” Grottols are prized as a rare source of diamonds. But only the quickest of adventurers can manage to catch them before they burrow away; grottols are very fast and surprisingly clever, even using minecarts to escape pursuers. Keep in mind that only iron or stronger pickaxes can penetrate their diamond shells.

(LAN-turn)

Lanterns dot the canopy of roofed forests, glowing like jellyfish. Some cultures believe that these creatures contain the souls of the recently deceased. Others wish on them like stars. Adventurers, however, know that the luminous jelly inside lanterns is a great light snack that will briefly illuminate the darkest of nights.

(NAH-guh)

Coastal cliffs are dangerous places. Adventurers tell of winged serpents called nagas with impressive speed and menacing acid venom. Though these drakes are evasive, a well-placed arrow can knock a naga right out of the sky; time your shots to hit when the naga is preparing an attack, and you can defeat it with ease.

Naga acid is deadly, but can actually be used to neutralize other poisons. The fangs they drop upon death can be used to brew poison antidotes and preventatives.

Strongholder - Floating gauntlet that grabs, pounds, and swats to attack. Floats eerily through stronghold tunnels, maybe passing through walls.
Phoben - Fear spirit that spawns in roofed forests. Wields fear magic, a lantern, and a long, darkness-infused blade called Ej. Fighting the phoben will involve carefully but quickly deciding when to strike and when to block.
The Sculptor - A simian guru that can be found meditating on mountaintops. He will challenge players to complete obstacle courses made of floating rocks. Those who pass this test are granted the ability to wield powerful earth magic.
Forest Dryad - Will grant players her magic pan flute in exchange for the Phoben's lantern as thanks for defending the forest.
Biloko - Rival tribe of the Barakoa. Jungle pack hunters that lurk in the canopy before leaping down on their prey. Their simian features, ornamented headdresses, long claws, and glowing red eyes strike fear into their prey.
Nimbus Drakes - Flying serpents with clouds billowing from its head and wings. The clouds turn grey before the drake unleashes its lightning attacks. It circles an egg in a nest on the top of its mountain, which can be claimed and hatched by the player who defeats the mother.
The Winter Order - A league of cryomancers residing in a frozen citadel.

Considering:
Large, scary insects in swamps, maybe large, scary frog-things to go with them
Anglerfish-like deep sea monster
Ents that can be created from trees using an item.
Golems that can be created from Geomancy boulders.
Snake Charmer - Humanoid desert boss that spawns in a temple structure. He can summon snake spirits by playing his flute, and can transform into a moving painting on the wall to dodge attacks and move across the room.
Just a plain dragon. I'd like to try my hand at this timeless monster concept.

I'd like to add at least one mob for each of Minecraft's environments, and I do like suggestions! BUT I'm very picky about what I work on. Only suggest your mob if you can imagine players thinking, "I can't believe I ever played Minecraft without this in my game!". I will not add monsters from existing franchises, or just giant versions of real animals. I'm not interested in creatures from existing mythologies unless I have a really original take on it. No griefing or one-hit-kill moves.