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"Flashback [cost]" means "You may cast this card from your graveyard by paying [cost] rather than paying its mana cost" and "If the flashback cost was paid, exile this card instead of putting it anywhere else any time it would leave the stack."

"Saddle N" means "Tap any number of other untapped creatures you control with total power N or greater: This permanent becomes saddled until end of turn. Activate only as a sorcery."

"Saddled" isn't an ability that a creature has. It's just something true about that creature. It won't stop being saddled until the turn ends or it leaves the battlefield.

A spell cast using flashback will always be exiled afterward, whether it resolves, is countered, or leaves the stack in some other way.

An ability that triggers when a creature "attacks while saddled" will trigger only if that creature was saddled when it was declared as an attacker.

Creatures with saddle can attack or block as normal even if they aren't saddled.

If a card has multiple instances of flashback, you may choose any of its flashback costs to pay.

If a card with no mana cost gains flashback equal to its mana cost, it has no flashback cost. It can't be cast this way.

If a permanent becomes a copy of a saddled Mount, the copy won't be saddled.

If you cast a spell with flashback, you can't pay any alternative costs such as overload costs. You can pay additional costs such as kicker costs. If the spell has any mandatory additional costs, you must pay those to cast the spell with flashback.

If you cast an instant or sorcery with {X} in its mana cost and a flashback cost equal to its mana cost, you still choose the value of X as part of casting the spell and paying that cost. If that spell has flashback {0}, you won't be able to choose the value of X; the value of X will simply be 0.

To determine the total cost of a spell, start with the mana cost or alternative cost (such as a flashback cost) you're paying, add any cost increases, then apply any cost reductions. The mana value of the spell is determined only by its mana cost, no matter what the total cost to cast the spell was.

You can cast a spell using flashback even if it was somehow put into your graveyard without having been cast.

You may activate a permanent's saddle ability even if that permanent is already saddled.

You must still follow any timing restrictions and permissions, including those based on the card's type. For instance, you can cast a sorcery using flashback only when you could normally cast a sorcery.