This hulking beast was originally bred by the dwarven Shaperate as a beast of burden and food source, the rough equivalent to surface oxen and cows. Some versions of bronto have even been developed as dwarven mounts, valued far more for their sure-footedness and stamina than for their speed. While present within Orzammar in large numbers, some bronto still exist in packs within the Deep Roads, having returned to a wild state after the fall of the dwarven kingdoms. They require remarkably little sustenance, consuming organic material from water, fungus and even rocks (hence the "rock-licker" appellation used by many dwarves to describe bronto), and exist in primarily dormant states until provoked. An angry, charging bronto is considered to be a rather dangerous opponent.
"There's only two things a noble will step aside for: Paragons and angry brontos."
—From Codex entry: Bronto
The bronto found in the Deep Roads during the Paragon of Her Kind quest seems to be actively working with the nearby darkspawn. Although it is not specified, it is likely that it is a ghoul, corrupted by their taint.
A bronto is a huge rhino-like beast based on the real world Brontotheriidae (a mammal that lived more than 10000 years ago). (Latin name: Bellum Iumentum - "War Beast of burden").