W00t! Blizzard added the Baneling for the Zerg in their official site, after a long time of sleeping (note that we had seen it in gameplay videos a long time ago)!

Here is the official link

Here is the short version:

he baneling moves itself by tucking into a tight ball and rolling. When a baneling gets close enough to an enemy, it explodes with devastating force and shower the immediate surroundings with searing acid. The baneling is destroyed in the explosion, which is very likely to inflict a huge amount of damage.

The Baneling is of course a morph of the zergling, the most numerous zerg subtype. Zerglings have been seen entering a brief chrysalis phase before emerging in their new form.

They seem perfect for the first line of attack, followed by ranged and melee units. Some community members call the Baneling "the scourge of the ground"