Magma Cube

Magma Cubes are hostile mobs found around The Nether. They will be released in Minecraft Beta 1.9. When killed, they leave smaller versions of themselves, then dying and leaving Experience Orbs (Much like 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 sink in lava, and are slowed by it, but do not drown in lava. They can drown in water, however.

Trivia

 * Magma Cubes can not jump out of lava if the lava is too deep but they will not die/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 quite abundant in the new Nether Ruins that were added by Jeb, contrary to normal slimes, which are quite rare.
 * Every Magma Cube size drops experience orbs, not only the smallest cube.
 * Magma Cubes currently make no sound, though C418 has confirmed he is working on one, as mentioned here on his twitter: http://twitter.com/#!/C418/status/118716572687810560.
 * 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.

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 smp.