Pretty poor from the dev team. Anyone not stupid could see you had two options.
1) Take it away from anyone who wrongly got it, and reward it to those who deserve it.
2) If your Neanderthal coders cant figure out how to reclaim the mounts from those who don't deserve it. Just give it to everyone and make an entirely new mount for the pvpers who deserved it. Give them a choice of a few designs and get them to choose their favorite. Most are happy to wait if they've got a nexu in the meantime.
Honestly the worst way you could've gone about fixing this is what you have done. Making the ranked players unhappy as they no longer have an exclusive season 5 mount. And made everyone else unhappy that their friends got a shiny new mount for doing nothing, while they are forgotton about.
0/10 for BW.