(Sorry for my English)
I know that Minecraft will get some updates soon, and it contains some mobs, but there's my suggestions for unique creatures that will add some more uniqueness into the game. Frist of them is today, and later will be some other ones.
1. Rodent (Mouse, Hamster, Rat, etc.)
HEALTH: 1,5 hearts
SPAWNS IN BIOMES: Plains, Forest, Savanna, Desert, Taiga, Tundra
DROP: 0 experience, but 1 Rodent Hide (actually an equivalent for Rabbit Hide) and 0-3 Seeds (random ones)
Rodents are small passive mobs that spawns in different biomes. In Plains and Forest it will look like a mouse, in Savanna - like hamster, in Desert - like gerbil, in Taiga - like vole, and in Tundra - like lemming. Rodents usually just run around the world, but if they see player, zombie (or any it's variant), creeper or cat, they will panic and run away until getting to any Grass, Sand, Dirt, Soul Sand, Gravel or Wool block (or any variants like Podzol or Red Sand). If it stands on block like this, or gets on it while running, it will dig fastly and soon will disappear, and block will be replaced with Rodent Hole. It will be unbreakable, but you can use shovel to dig it. After digging about 45-50 seconds, hole will be replaced to block that it was before, and rodent will appear and run away. Also rodents can jump very high, about 2 blocks in height, and climb ome of the blocks, so they can get even on trees and some structures such as mansions.
Rodents will pick up and eat some food from ground, like Cakes, Seeds, Bread, Apples, etc, even things unedible for player, like Nether Wart or Hay Bale. Also they will steal it from chests and shulker boxes so they will be dangerous for your provisions! If you will sleep at night, and in radius of 16x16 blocks from you will be rodent and food in your inventory/chest/shulker box/etc, 2-6 food will disappear. Also rodents will automatically run to any planted crops, like beetroot, carrot, potato, wheat, nether wart, pumpkin, melon, chorus, mushrooms or sugar cane (crops may be planted by you or villagers, but not just generated in the world), and after finding the crop, rodent will break it and eat all the drop. Sometimes rodents will have "raids" on farms (basically chance is around 2-3%, but with every one planted crop it will increase for 0,5-3% (depends on the crop and game difficulty). Raids usually happens at night or in dark places. When raid happens, on distance of 80x80 blocks from the random player's or villager's farm will spawn a huge amount of rodents (about of 7-22) and them will run to farm and eat any crop that they find. After "finishing" one farm they will go to next and next, not randomly, but purposefully, until they will "clean" about 16-24 farms. At that time they will run in the world randomly. At raids, rodents may destroy all player's farms, so you need to protect them.
Rodents can be a huge problem, but everyone knows the way to stop the mice - cat! Rodents will run away from cat if they see it, and cats will attack them. Rodents are fast, but cats can run even faster, and cats will easily chase them down. Also cats can dig them from Rodent Holes, and that takes for cat only about 10-15 seconds (3-5 times faster than player can with shovel!). After killing the rodent, tamed cat will pick up it's drop and go to player. After that, it will chase another rodent until they will end around the cat. And yes, if cat kills the rodent, it will drop not just hide and seeds, but with 100% one of these things:
Cats can be just sittings to "protect" from rodents - because they (rodents) will escape from cat if they see it. But using normally walking cats is a better way, because they will hunt and retrieve you the rodents' drop.
And yes... Rodents can be "tamed" for short time! If you feed them with pumpkin pie, they will follow you and don't be harmful for food and crops, but only for short time. It can be helpful for zoo building and etc, but in survival it is mostly for decoration... MOSTLY. When the "friend" effect ends, rodent will drop any random seed and run away. It may be helpful if you need to feed your chicken friends or to grow some pumpkins.
There's easter eggs:
If you name a rodent "Science", "Scientist" and any other related thing, it will look like a lab rat.
If you name a rodent "Spot" or Пятнышко", it will be black-and-white rat. When I studied at school, one of my teachers had a rat named Пятнышко (Spot), and it was black-and-white. She (teacher) was one of the best people I ever met, so I decided to perpetuate her rat in Minecraft, like Toast the rabbit.
If anyone would ask:
Why they should be added?
1. To add some more wildlife into the game.
2. To make players don't build the cheater farms.
3. To make cats more useful.
4. To make villages a bit less easy-to-live places.
DROP: 3 experience, Rodent Hide and 3-6 Nether Quartz piece.
Nether Rat is a "variant" of Rodents in Nether. They are much more dangerous than normal, because they can damage you. They will not attack you, but you will get damage if you stands on or next to them (like with cactus and magma). Basically, they appear in Nether, but very rare. Nether Rats look like normal rats, but they are red and black with flaming tail. They behave like normal Rodents, but instead of running from you they will not do that, digging into the holes and running away only if Cat or Creeper is close. Also they will dig only into Soulsand, Netherrack and Glowstone. They will, like normal rodents, search for chests with food and farms, but their raids are 2 times smaller - instead of 7-22, Nether Rats' raid will be just 4-11. Way to protect from them is similar to normal ones - cats! Cats will not take damage from Nether Rats and will kill the "fire rodents" with one hit. Unlike normal Rodents, Nether Rats will not be "tamable" for short time.
Why should they be added?
1. To add more life to forgotten Nether.
2. To make lesser cheater farms.
Normal Bite (land): 4 hearts (easy, normal), 5 hearts (hard)
Death Roll (water): one hit kill to most mobs, except some ones - to them 12,5 hearts of damage
Grab: 1 heart
SPAWNS IN BIOMES: Swamps, Lukewarm and Warm Oceans, Beaches, Deep Oceans, Rivers in Desert, Taiga, Savanna and Jungle.
DROP: 9 experience, 0-2 Crocodile hide (a new item!), 2-4 Crocodile teeth (also new item)
Crocodiles are a new danger to player and to most peaceful mobs! They spawn next to the water and have unique types - dark green Alligator (taiga), brown and sandy Nile crocodile (desert, savanna), bluish-green Saltwater crocodile (oceans, beaches, deep oceans), brownish-green Mugger crocodile (swamps), blackish-gray Black Caiman and bright green Jacara Caiman (jungles). Crocodiles are aggressive and will attack player and also many mobs - cows, chickens, sheep, pigs, horses, villagers, zombies, ocelots, wolves, skeletons, spiders, fish (except Pufferfish), squids, turtles, dolphins, polar bears, llamas, silverfishes, illagers, rabbits, golems, endermen and variants of these mobs (strays, husks, witches, etc.). They are dangerous on land, because they 3-hit the player on easy and normal difficulty and 2-hit on hard. Also they have 4 defence points due to their tough skin. Their weakness is speed. Crocodiles are slower than player and bow is a best weapon against them for now. But never fight crocodile in water! It will do it's Death Roll attack. It will one hit kill most mobs, except Wither, Ender Dragon, Guardians, Llamas, Pigzombies and Endermen - they are too "hard" for them, and it will deal just 7 hearts of damage to them (but it still one hit golems!). In water, crocodile is the most deadly mob, so never get close to it underwater. Also crocodiles can grab and drag underwater chickens, silverfishes, pigs, sheep, rodents and rabbits. The only thing that makes them less dangerous that they could be is their rareness.
The crocodiles are extremely hard to fight, but their drop is good. Crocodile hide is good to create armor, armor from it is a bit weaker than Diamond, but it lets you swim in water faster. Crocodile Teeth can be used to make a Crocodagger, a knife\sword thing that deals low damage (about 1,5 - 2 hearts) but lets you drag mobs where you can (right click) or Pierce them, impaling on sword for 20-40 seconds and dealing damage with time. And yes, crocodagger breaks slower than diamond sword and can be used to cut and get vines and leaves, but not sheep wool. Also from teeth you can craft Tooth Arrows that will deal more damage than normal ones.
Crocodiles can be tamed. To do that, you need to feed them some rare food - Eye of Ender, Spider Eye, Tropical Fish, Nautilus Shell, Rabbit Stew or Rabbit's Foot, by dropping it out of inventory. Crocodile will be tamed for 6 minutes 20 seconds, and in that time it will behave like tamed wolf, can be ordered to sit or stand, follow you and protect you, and can even be ridden with saddle! Riding is perfect underwater, where by pressing X you can use Death Roll to fight. After that time crocodile will crawl away, and after 2 minutes from that moment it will become hostile again.
Also crocodiles (only tamed) can be bred. After breeding they will lay eggs like turtles, and baby crocodiles will appear from eggs after some game days, they will be wild but not hostile until they grow. After growing they will drop 2-3 Crocodile Teeth like turtles drop Scutes after growing.
And yes, there's two crocodile-related achievements. They're called:
1. THE WAR OF BEASTS
To do that, you need to tame a crocodile and fight Polar Bear, Iron Golem, Guardian (or Elder Guardian), Illager Beast or any boss.
2. URBAN LEGEND
To do that, you need to tame a crocodile and drag it to Stronghold or to jail in Woodland Mansion. It is based on urban legends of "sewer alligators" that then crawled to houses or streets and attacked people.
WHY THEY SHOULD BE ADDED?
1. To make Minecraft a bit more hardcore
2. To add more wildlife
3. To add more life into forgotten Swamps and Deserts
4. To make oceans and swamps more dangerous
5. To finally add an apex predator into Minecraft)
6. To make unique weapons and armor.