There is a legendary mount scroll you can get on the Rabbini traker npc at Hakain's Crossing, Victory Plaza. However be warned this is NOT a bestiary scroll, meaning it will not add the mount to your bestiary, it will only turn a Legendary pet into a mount.
I went into derp mode and wasted 40 rabbini coins trying to get a legendary count on my bestiary. (since I wasted 3 marks before QQ) and of course I got the mount but not the tame count on the bestiary. To be fair with Nexon/Wemade though, it was my mistake for not reading the description.