The kaboomer appears in the multiplayer map "Left 2 Die". It is a far slower, larger and more effective version of the baneling. While the baneling is a small and fast suicide unit, the kaboomer is capable of dishing out considerable damage to bunkers or other defenses in the form of an acid spew before dying.
The kaboomer's abilities include spewing acid continuously (especially effective against buildings), saturating an area with acid (causing area of effect damage in the targeted radius), and exploding in a similar way to a baneling, although the area and damage are greatly increased.
Kaboomers appear as special infested on the "Dead of Night" mission in Co-op Missions. They are slow moving area of effect attackers. Kaboomers also deal area of effect damage when killed.
As Left 2 Die is based on Left 4 Dead, the kaboomer is likely based on the Boomer or the Spitter.