"The baneling is a creature so bloated with fluid-filled sacs that it can barely walk; instead, it moves itself by tucking into a tight ball and rolling. However, this ungainly appearance belies the fact that the baneling is an extremely dangerous organism, one of several new zerg specialists recently seen on the battlefield. When a baneling gets close enough to an enemy, the creature triggers a reaction within its volatile chemical payload that causes it to explode with devastating force and shower the immediate surroundings with searing acid."
Another pic: Large
Reference: SC2- Baneling