In all seriousness, a true "monster manual" would be just a list of people who leave their shopping carts in the parking lot instead of returning them to the collection spot.
Taken.
Casual Shopper
Medium Respectable friendly brother (formerly known as Monstrosities), chaotic evil
Armor Class 10
Hit points : 4
Speed : 10 ft (when puting their things out of their bags at the cashier) 90 ft (when trying to snatch the last item on sale just after you've seen it)
STR 10 DEX 4 CON 10 INT 4 WIS 10 CHA 8
Skills: Perception +4 (to spot said last item), disadvantage on Use Tool (shopping cart)
Language: you can cuss at them in any language you want.
CR 1/8
Actions:
Incite rage: the Casual Shopper shopping habits cause onlookers to be at disadvantage on any action except hitting them as long as they stay within the shop.
Rush hours: removing a casual shopper won't solve the problem as two more casual shopper will spawn on the next round.
Use discount coupon: using a coupon to pay, a casual shopper can cause an entire quue to be Incapacitated if they fail a Wisdom save, DC 16, to change line in time.
Lair actions:
Inside a Walmart, can summon a Walmart Greeter (recharge on 2-6)