If a creature has been Polymorphed into a beast, can it still use an already summoned Dancing Sword to attack?
The target's game statistics, including mental ability scores, are replaced by the statistics of the chosen beast. It retains its alignment and personality.
The creature is limited in the actions it can perform by the nature of its new form, and it can't speak, cast spells, or take any other action that requires hands or speech.
The target's gear melds into the new form. The creature can't activate, use, wield, or otherwise benefit from any of its equipment.
Dancing Sword says:
While the sword hovers, you can use a bonus action to cause it to fly up to 30 feet to another spot within 30 feet of you. As part of the same bonus action, you can cause the sword to attack one creature within 5 feet of it.