Prior to patch 7.2 I had 249 mounts. I was working on number 250 for the achievement when 7.2 hit, and when I logged in I saw I suddenly had 248 mounts. I logged into my demon hunter (with the class mount) and my hunter (with a leather working mount) to see if that would shake things loose. Alas, I was stuck at 248 mounts, with no idea which one got lost in the shuffle.
Yesterday I finally completed Glory of the Draenor Hero, after letting it go for a long time. I got stuck doing the achievement in the Shadowmoon Burial Grounds where you have to step on the runes during the Saldana fight, and the achievement in Upper Blackrock Spire where you have to kill the Orebender without allowing him to cast a certain spell 4 times. Well, as a fresh 110 a few months back I did not have the required DPS to kill the Orebender, and I tried the other achievement several times without success. Fast forward to now, at a decent ilevel, and I was able to kill Orebender with help in a couple seconds flat, and finally got the Saldana achievement (turns out I was stepping on the wrong runes), and got the boar mount for my troubles. It should have been mount number 249.
Nope. My counter still read 248.
Last night I logged into my warlock, who is a tailor with several magic carpets, and boom!
Took me long enough.