Pitiless Horde

If a creature enters the battlefield as a copy of or becomes a copy of a creature whose dash cost was paid, the copy won’t have haste and won’t be returned to its owner’s hand.

You don’t have to attack with the creature with dash unless another ability says you do.

If you choose to pay the dash cost rather than the mana cost, you’re still casting the spell. It goes on the stack and can be responded to and countered. You can cast a creature spell for its dash cost only when you otherwise could cast that creature spell. Most of the time, this means during your main phase when the stack is empty.