First, the card itself. It costs 3 mana to cast (2U), which is a fair cost considering what this card does. Upon casting it, you choose a creature on the battlefield. This ability doesn't even target, so shroud and hexproof can't stop you from choosing a creature. You can then make any number of target creatures you control into a copy of that creature until end of turn. You do have to pay 1U for each target beyond the first. But it can become quite a blowout if you're copying the right creature!
This ability to target multiple creatures was a mechanic called Strive, one that's exclusive to the Journey Into Nyx set. There are other spells with Strive that would go on to see some amount of tournament play. This looked like it would be one of them.
Did Polymorphous Rush Ever Enable Combos in Standard?
People were very excited when this card was spoiled for Journey Into Nyx, especially because of the Strive ability tacked on to it. Polymorphous Rush seemed like a toolbox card that could work well on offense and defense. While it can end up being a lot of mana for this card to work well, Mono-Blue Devotion, one of the premier decks at the time this card was released, made plenty of mana. It looked like a great way to get out an extra Master of Waves in Mono-Blue Devotion! It could copy a Blood Baron of Vizkopa several times and make your opponent's life miserable!
Also, Heroic was a big mechanic in Standard due to the Theros block. Playing a spell that could target multiple creatures such as Polymorphous Rush seemed awesome! It would get the heroic ability first, such as gaining a +1/+1 counter, then become that creature. That works well in theory.
There was also a cute combo with Sage of Hours and Hero of Leina Tower. Essentially, you'd copy a bunch of Heroes and turn them into Sage of Hours. You could then pay X to add extra +1/+1 counters to the Heroes, and then use Sage of Hours ability to take a bunch of extra turns. Once the Heroes turned back into themselves, you could just beat down with them during all those extra turns. Worked well in theory, but it seemed a bit janky even at the time.
But it never really took off in Standard. Not even in the battlecruiser format of Commander where there's plenty of mana. What happened?
The problem is pretty much this: it really just ended up being more mana than it was worth. Another issue is that you don't get the enter the battlefield effects of the creature it copies. The decks that really wanted this were those that wanted to get extra copies of Master of Waves, but ones that came along with tokens, not just a +1/+1 boost for the Elementals. And yes, it could be an awesome card to copy powerful creatures on your opponent's side of the board, such as the aforementioned Blood Baron of Vizkopa, one of the more powerful creatures that has been in any Standard environment.
But even if it never cracked its way into competitive "top 8" Magic, there were other ways that more casual players could use it, right?
One deck that screamed Polymorphous Rush to me at the time was Biovisionary Combo. One of the more fun win conditions available in Magic the Gathering, Biovisionary gives you the chance to win the game at the end of any turn if you happen to control four or more creatures named Biovisionary. There are numerous ways to do this, obviously, but Polymorphous Rush became the easiest method available in Standard. With the addition of Battlefield Thaumaturge, also in Journey into Nyx, Strive costs became a lot more palatable. The Thaumaturge would make an instant or sorcery spell cost 1 less colorless mana to cast for each card that it targeted beyond the first. It seemed pretty good.
But wait, you need 4 Biovisionaries at your upkeep to win the game! Not a problem. Because Polymorphous Rush can be played at instant speed, you can do it at an opponent's end step, which is silly. With how quickly Simic (blue/green) decks can ramp, it looked like this could be a decent Tier 2 deck until the October 2014 rotation. I wasn't alone in feeling that Biovisionary, Rush, and Thaumaturgee were made for each other. I felt that this combo could even slip into fringe Modern play-ability with the existence of Mirrorweave from Shadowmoor.
While this worked in theory, setting up the combo really just never worked in Standard. It proved to be far too easy to disrupt. However, a Biovisionary deck would in fact work eventually. In 2016, SaffronOlive of MTG Goldfish would make a Biovisionary deck that worked in his Against the Odds series. He went 0-8 in his first attempt using Mirrorweave. But in his second attempt, which utilized Collected Company, it didn't even need Biovisionary to win. Funny, right? And no, there were no copies of Polymorphous Rush in the deck.
There was another thought about Polymorphous Rush. What about using it alongside Young Pyromancer? People were already trying out Battlefield Thaumaturge in Pyromancer decks, too. Young Pyromancer is an awesome creature, and he still sees play in Modern, Legacy, and Vintage today. But sadly, while you could turn the Elemental tokens from the Pyromancer into other Young Pyromancers, Polymorphous Rush itself wouldn't make extra copies from the new Pyromancers. This is backed up the official rulings on Polymorphous Rush from Wizards of the Coast.
Even then, it seemed like Rush would still see play to make more Pyromancers, since there were plenty of cheap spells that would immediately help you benefit from the extra Pyromancers. In theory, this was a good idea, but who wanted to spend that much mana to not get tokens off of the new Pyromancers?
So it was a non-bo.
What About Commander, Where Fun Cards Like This Live On?
Polymorphous Rush never caught on in Commander. Usually, that's a format where a lot of cards with splashy effects like this live on. It does see play, but it's very scattered. So due to it not really being played outside of a handful of random Commander decks, its price has remained that of a bulk rare. You can find copies for about $0.50 USD or less, but due to the casual appeal of the card, some retailers may ask as much as $1 a copy.
Even then, you should definitely keep this card in mind if you're already considering using it. You can use the existing combos in a Commander deck that you might have already. It's just more mana-intensive in practice that you might at first realize, for any of these, and even with the help of the Thaumaturge.
So yes, Polymorphous Rush does work with Biovisionary as you might expect. It works with a lot of cards. But in all of these cards, it's been proven to be too slow to be effective on a consistent basis. It's still neat. And its past history doesn't mean that you won't ever see it become part of a broken combo, though. That it hasn't happened yet doesn't mean anything. Wizards just hasn't printed the card that it will break just yet.