They appear to be regular tanks that are green like slime. They also have two headlight at the front.
When Finn and Slime Princess began decided to fight against Blargetha to stop the trials, Blargetha climbed into one of the tanks and attempted to shoot them. It was then revealed that instead of regular explosives, the tanks were manned with explosive slime balls.
- Although they look green, they don't appear to be slime based like most other things in the kingdom.