ok I just transfer from my white horse to my tiger and was skeptical because it show 2 stars for the tiger and 1 star that was grayed out. after paying 5m for the transfer it added to the existing XP of tiger and there no deductions (other than the 30% because I couldn't afford to add gems). so in the above example from my OP it would be: mount A got 10k XP and mount B got 5k XP. when transferring from A to B will B have 12k XP (70%: 7k+5k)



edit:
Quote Originally Posted by JOEANOMALY_7643381 View Post
It's like a hero transfer...it knocks the source mount to zero.
that wasn't the question, we are all well aware that the source will have 0 XP after the transfer