Magma Cube

Magma Cubes are hostile mobs found around The Nether. When killed, they release 2-4 smaller versions of themselves in addition to Experience Orbs, similar to Slimes. When they move, they jump up and momentarily unravel into several layers, giving them the appearance of a spring and revealing a molten lava core inside. They are slowed by and sink in lava, but do not drown in it.

Behavior
In their passive state, Magma Cubes move slowly, jumping forward every two seconds or so. Upon spotting a player, magma cubes will jump towards them more quickly, but the player can still outrun them. It seems that Magma Cubes try to eat the player. Also, they will try to damage you by jumping directly on top of you. However, they cannot damage other mobs like this.

History
Magma cubes were first seen in Minecraft 1.9 Beta Pre-Release 1.

They were also added in the full release of Minecraft.

Prior to Minecraft 1.0 RC1, Magma Cubes did not make sounds.

Trivia

 * Magma Cubes cannot swim in lava, only jump along the bottom. Thus, they can get trapped in deep lava, although they will not drown.
 * Magma Cubes seem to be the Nether equivalent of Slimes. Although they appear similar, the components of their texture files are quite different.
 * Contrary to popular belief, Magma Cubes are actually less abundant in the Nether Fortresses than in the rest of the Nether due to fewer large flat surfaces to spawn on.
 * Every Magma Cube size drops experience orbs, not only the smallest cube. This means that killing a huge cube and all its spawnlings would grant the player 28 experience, as opposed to 5 experience of other hostile mobs.
 * Despite trying, Magma Cubes are unable to jump in water, although they can move around. They will drown if completely submerged.
 * Unlike tiny slimes, tiny Magma Cubes can still hurt the player.
 * Unlike Slimes, which are rare and only appear below layer 40 in certain chunks of the normal world, Magma Cubes can appear anywhere in the Nether.
 * Magma cubes are often found in lava, since they are unaffected by it.
 * Like Slimes, when a magma cube is not chasing the player it will hop in one direction.
 * Magma cubes can be spawned in the overworld if a spawner is hacked to spawn them. They do not require netherrack.
 * It seems that Magma cubes (or at least tiny ones) can only damage you using their bottom side.

Bugs

 * Magma Cubes do not seem to use their spring-like jump effect in multiplayer.
 * They also seem to frequently glitch through walls, ceilings, and floors, to attack players on the other side; dropping down through the solid roof of a tunnel, for example, then glitching back up to the roof out of sword reach. Or they will lunge out through the walls of a tower while a player is descending via a gravel pillar, and push them off said pillar without causing attack damage.  They also seem to sink into Soul Sand, being invulnerable while merged inside a Soul Sand block, yet being able to jump up and attack from inside their protective glitch.