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.

“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.

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

The value of X is determined as Stubborn Burrowfiend’s triggered ability resolves.

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

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

“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.”