not your fault, doesn't matter which server we play,
i also doubt lag will cause that kinda of harm,
i believe you can paid, you can have the mount(unless Term of Agreement state otherwise)
you should ask the official Blizzard Tech service for more information, or maybe they will help fix this
Beware ramdon blizzard look alike hack website.
or you could even open an in-game ticket for help(will take a long time, 2~3 days maybe, mind you)

on the personal note, i find this purchase mount a waste of money, but thats my opinion