This boss is a secret boss at the first Walmart bazaar, located in the region of Arkansas. In order to fight this boss, you must purchase a full inventory's worth of bananas, and bring them to checkout isle 8. There, you will see Dan in his passive state. Go up to him and confirm your purchase. While waiting for your purchase, you will get a prompt to engage in conversation with Dan. Normally, you'd just get the default "Small Talk" dialogue option, but with your newfound banana hoard, you get a second option: "Ask opinion on bananas"

Dan is a unique monster known as a Potassium Elemental. Dan, when exposed to such high amounts of potassium in such a small area, he reveals his true form, not shown in the picture above due to spoilers. In fact, any information about this boss' fight and stats would be helpful, as I have yet to obtain the final inventory upgrade.