Magma Cube

Magma Cubes are hostile mobs found around The Nether. They will be released in Minecraft Beta 1.9. When killed, they release 2-4 smaller versions of themselves in addition to Experience Orbs, slightly unlike Slimes, which split into 4. 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. Spotting the player causes them to jump towards him/her more quickly, but they are still easily outrun. Unlike most aggressive mobs, Magma Cubes do not damage you on contact. They can push the player harmlessly. Instead, they will try to damage you by jumping directly on top of you. When they land, they create fire which can damage you. However, they cannot damage other mobs like this.

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 .pngs are quite different.
 * Magma Cubes are less abundant in the Nether Ruins 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 makes it a good target for people who want experience.
 * Magma Cubes currently make no sound, though C418 has confirmed he is working on one.
 * 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.
 * Due to the high amount of experience points they drop, magma cubes are a viable slaying option if a player wants to try high-level Enchanting. However, the experience orbs from Magma Cubes seem to fill the experience bar far less than those of, for example, a Blaze.
 * Unlike Slimes, which are very rare and only appear below layer 20 in the normal world, Magma Cubes appear frequently and everywhere in the Nether.

Bugs

 * Much like slimes, magma cubes do not despawn when the difficulty is changed to peaceful.
 * Magma Cubes do not seem to use their spring-like jump effect in multiplayer.