My mount is missing

I had a lot of mounts and got the achievement of 100 mounts and you can see it if you want but my mount is missing it says i got the reward but i'm not able to see it neither use it. Can i have my mount back ? Also i can't use my Grand Ice Mammoth which i got my my horde death knight before a year and i transferred the horde character to alliance and i was able to use the mount before(the horde mount on the Alliance) but now i can't if you can make me use it again i'd be very thankful.
Thanks in advance! <3