The AT-PT was a single-trooper policing unit that existed well before the Clone Wars. While many resources establish it as a Republic vehicle, it was a common infantry vehicle in militaries across the galaxy.
The AT-PT was only 3 m tall and could only fit one soldier inside; nevertheless, it was fast, cheap, and decently armed for a vehicle of its size. When the Clone Wars escalated, most of these vehicles were junked as outdated models and replaced with more advanced walkers. This caused many of them to fall into the hands of private militias and gangs, who used them to incite their own rebellions.