O Gitrog, Montaria Voraz

"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.

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

Putting one or more land cards onto the battlefield with The Gitrog's triggered ability doesn't count as playing a land. It can put land cards onto the battlefield even if you've already played your land for the turn.

Use the power of the sacrificed creature as it last existed on the battlefield to determine the value of X.

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