The All Terrain Open Transport was a walker used by the Republic forces during the Clone Wars. It epitomized everything about Republic war machines: Big, heavily armed, and extremely vulnerable. Despite this, the Emperor was actually disappointed with the final design because the engineers had removed 24 legs from his initial idea.
Luke Skywalker was thinking about telling his fellow pilots at Yavin Base "I used to bullseye stupidly exposed stormtroopers in AT-OT's with my T-16 back home." However, he realized that he would probably be laughed at for telling a tall tale, so he changed it to the more acceptable "I used to bullseye womprats in my T-16 back home"