During the Zombie Apocalypse, Crazy Dave was cultivating more and more species of sentient plants. One day he thought:
"I have so many plants and experience with them in my collection. Why not create a new one myself then? Because I'm CRAAAAAZY, of course!"
He thought of war equipment that humans used way before the Zombie Apocalypse, and first thing that came to his mind were tanks. Since Peashooters were his favourite and most common plant, Crazy Dave decided to merge those plants with tanks by making a partially liquid Peashooter species and by replacing track with legs in tank, then putting both prepared ingredients together. Even after noticing some mutations like floppy ears or a Mini Pea that's unable to shoot, Peashooter Tanks were warmly welcomed by rest of the garden.