Fully mutated, and completely savage.
Burnouts have seem to lost all traces of sapience, and (definetly) humanity.
They also seem to have lost self-preservation, since they mindlessly charge at their prey, without a second thought.
Aside from being extremely ferocious and an insatisfiable appetite for human flesh, Burnouts also have rock-hard skin, and the only way (well, easiest way) to kill them is to shoot the orange glowing growths that cover their bodies. Fortunely, they're easy to hit, and they're still vulnerable to explosives, and environmental hazards, such as crushing gears, giant cacti, and meat mincers.
The Original Burnouts are from Stygia. While the Prisoners, who worked in the toxic infested underground, were going on a massive rampage through the city, the majority of the tourists when down into the underground to hide, not realizing that the caverns were filled with Gamma Radiation, from the city's radiation filters.
Being down there for too long, and toxic waste everywhere, it should be kinda obvious what happened.
It's not known where these new Burnouts are coming from, but there may be a massive spill of Gamma Radiation somewhere that may have twisted a large population of people.