Flamerush Rider

If the copied creature was copying something else, the token enters the battlefield as whatever that creature was copying.

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.

If the copied creature had {X} in its mana cost, X is 0.

Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any "As [this creature] enters the battlefield" or "[This creature] enters the battlefield with" abilities of the copied creature will also work.

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.

If the copied creature is a token, the token created by Flamerush Rider copies the original characteristics of that token as stated by the effect that put it onto the battlefield.

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

You choose which opponent or opposing planeswalker the token is attacking as you put it onto the battlefield. It doesn't have to be the same player or planeswalker Flamerush Rider is attacking.

Although the token is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example).