I intuit crazy nuances. Sometimes bugs, sometimes just oddities. This is one of those oddities.
I polymorphed a mount. The rider stayed mounted. On the turtle.
This is WotLK content, and the equivalent Cataclysm content would be in Deepholm. To the west there are some mounted troggs. If polymorphed, they act as expected. If approached, the mount spawns and attacks and the rider stays polymorphed.
In this case, the rider stays mounted, and stays aggressive.
I like the idea of an undead riding a cute little turtle.
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