Magma Cube

Magma Cubes are a hostile mob 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. 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.
 * Magma Cubes seem to be the Nether equivalent of Slimes. Although they appear similarly, the components of their .pngs are quite different.
 * Magma Cubes are quite abundant in the new Nether Ruins that were added by Jeb.