Originally Posted by Timmie
That's true for Trine, but Dragon Zombies can be fought any number of times as a random encounter by exploring one of the two paths you're prompted to choose between near the end of the Northern Crater (the right path).
But as far as I know, Trine can only be learned from Yuffie's father at the top of the Wutai pagoda, or from the Stilvas during the Great Glacier event.
And Edit:
I was looked through the FFShrine link above, and I'm not sure how accurate the information regarding Pandora's Box is. I thought I remembered times where I've fought many Dragon Zombies before learning the enemy skill, but I might be wrong. Better follow the FFShrine advice just in case.