I did it last year with some wedding castles, rc cars, and large cadances. This year i did it with some princess palace playsets and mini figure sets. You have to have proof to do it like an add or photo, i dont know if online prices from other stores will work for walmarts price match. My managers know i collect ponies so they trust me and mark ponies with rollback prices to compete with the other stores if i pull out my phone a.d show them something from toys r us that is cheaper! Right now I feel like at least in my area (aka my pony hunting range) tru and target's pony prices are either on-par or cheaper than the walmart ones.
I would check and see if there are any paper adds at toys r us, target, meijer, etc with ponies in it and show the walmart cashier at checkout when you are buying your ponies, show them another store has it cheaper then theycshould price override for you! If im not mistaken, you might be able to price match stuff youhave already bought at walmart if something goes on sale later you purchased and you get refunded the difference.