The Monsterbots were a trio of Autobot Transformers -- Doublecross, Repugnus, and Grotusque -- added to the toy line in 1987. Similar to the Dinobots before them, they all transformed from large robotic monsters into robots, and had the ability to "breathe fire" by shooting sparks from their mouths in monster mode.
The Monsterbots were never used in the Transformers cartoon, and were only used briefly in the Transformers comics during the "Headmasters" miniseries.