The new tank mounts are rewards for completely new Achievements. Basically every tank can get 2 mounts now. Check the Achievements under the Battle -> Battle tab and there you should fine up to 6 Achievements for each tank (if you haven't done some already). The new mounts are obtainable by doing 300 lv 61 and above dungeons, lv 61 or above extreme trials, 50/60 rlts and lvling rlts (last to only count daily) in total.