Mask Effects.

Pig - No longer need to eat, gain buffs from eating golden apples
Wolf - Higher chance of spawning wolves that will attack enemies
Ocelot - Gain rare loot from fishing
Rabbit - Jump boost  / nightvision higher with levels
Sheep - Extra wool while shearing sheep
Cow - Higher chance of removing debuff immune to blindness and nausea
Mooshroom - Stew heals 2.5x hearts
Horse - Speed higher with levels
Chicken - Chance of no fall damage
Silverfish - Double ore drops 3x3 mine at level 4 perm haste at level 5
Slime - Fall damage is reduced, higher chance of giving slowness, fall damage taken is turned into AOE
Endermite - Higher chance to gain invisibility on pearling, immune to pearl damage, cooldown is reduced, blocks mine go straight into inv (4+)
Cave Spider - higher chance of giving posion, higher chance of nausea / hunger, immune to poison
Spider - higher chance to poison enemy while hit / hitting them
Zombie - Rotten flesh does not give hunger , poison can heal, regenerate health faster at night
Skeleton - higher chance to give debuff when shot / shooting
Villager - Higher chance to drop double heads on a mob (manually killing)
Guardian - Higher chance to give enemies mining fatigue, infinite water breathing (5)
Magma Cube - immune to fire damage / lava, chance to heal
Ghast - Fire aspect last longer, chance to give weakness to enemies, regeneration and arrow immunity (5)
Pig Zombie - Chance to spawn 4x pigman when hit
Blaze - Arrows are fireballs, immune to fire damage / lava
Witch - Higher chance to give enemies debuffs when getting hit
Wither Skeleton - Arrows can give wither effect, blindness, strength II (5)
Enderman - Close to death gives speed IV, chance to teleport when hit by an arrow
Golem - Absorption
Creeper - Higher chance for an explosion to spawn nearby, when dying tnt spawns nearby.
Squid - ????
Skeleton Horse - ????
Zombie Horse - ????