this idea is also found on the competent mobs thread, as i suggested he does it, so i posted it here, and in suggestions
maybe some passive mobs get sick and tired of being pushed around, and evolve, the pig, chicken, cow, sheep, squid, and........errrr......AIR evolve into
BOAR: ooh, this is nasty, the pig evolves into a pissed off wild boar, he is brown and has tusks, runs faster, and charges. the Boar sports double health as the pig (10 hearts) and each charge does 2 1/2 hearts
EAGLE: Trots around on the ground, similar to the chicken it once was, but when the player nears it leaps into the air and hurdles toward him. the eagle does 2 hearts per charge, and has 6 hearts (3x ammount of chicken)
BULL: the Bull has horns 3x the size of the cows, and can usually be seen by other cattle, if provolked, he will let you know by sending a wallop of a hornbutt your way, sending you 4-8 blocks on the opposite direction. The Bull sports a whopping 15 hearts, and does 4-6 hearts of damage per charge, 1/2 a heart per hornbutt. the Bull also has a second attack, after a hornbutt or charge, you may get the wind knocked out of you, and you will not be able to get up, the length this lasts and if it happens at all depends on how long the charge or hornbutt propells you, if you go 8 blocks or over, you are vulneralbe to incapacitation, if you are unlucky enough to get knocked off a cliff by the bull, you are incapacitated for as long at it takes to mine an obsidian block, fortunately however, when put in your "vulnerable to knockdown" phase, you arre immune to fall damage (it may also be worth noting that if you go 8 blocks or over in ANY direction, you will be incapacitated, but you will be immune to fall damage, this includes going up and down) when downed a bull can "Gore" you, resulting in an instakill, but whether a bull will gore you or not depends on its nature, if a bull has Red eyes it is more likely to gore you when incapped, and will attack other mobs to protect him and his herd, Bulls with Blue Eyes are more likely to spare you when incapped, and even nudge you with their horns in an attempt to help you up
Goats: Goats, like they do in real life, eat everything, and i mean EVERYTHING, mushrooms, cakes, flowers, shrubs, sometimes torches, things left on the ground, redstone (aint that a *****?), and even try to nip you and damage ur armor (since we cant edit the skin to amke it look like hes eating the clothes very well can we? not EVERY miencraft skin is human, so we cant do that) Goats sport 8 hearts (3 more than sheep) and cannot be sheared, but upon death it drops some of the things it tried to eat (dont think to hard about how you're getting them) everything from dirt (probably not dirt) to diamonds, anything can come from this thing's gut
Giant Squids: (expecting cthulu werent you? toobad ) Giant squids are a huge, i mean HUGE counterpart to the squids currently ingame, they can grasp the player and apply deadly pressure, making death by lava look like euthinization in comparrison, but being huge, they are also quite rare, the giant squid can also smash his tentacles into the ground, making the effect simialr to a tnt explosion, or an orge from mo creatures. giant squids are no pushovers either, with 80 hearts (160 times normal squid) the only option is to try and escape the mutli-chunk sized beast and hope it despawns, and hope it dosent spawn near your ever so preacious water castle that all the ladies dig.
ok, ok, alot of people are going to bug me about this, i will do air too
Sulphuric Air this air can be found as the result of creeper explosions, or in lava caves, this air can commonly be found is caves, and can be created and weaponized by smelting gunpowder (which i think is already called sulpher ingame) and then crafting it with glass, creating a "Gas Grenade" (just dont think CoD 7) when thrown, the glass bottle containingthe gas explodes, spewing the substance everywhere, this gas poisons anything that walks through it, it stings the eyes (lucky skeleton bastards) and does 3 hearts per every 2 seconds in it, (im starting to rethink about it being in caves) it also impairs vision until you escape from it, fortunatly, it cannot kill the player, this prevents you from accidantly dropping one in a cave and gassing youself to death.