Despite posts by Blizzard that the Mythic Guldan mount (http://www.wowhead.com/item=137575) is a guaranteed drop, it is only guaranteed if you have 20 persons in the raid. Blizzard's previous statement on this being guaranteed was posted here:
https://twitter.com/warcraftdevs/sta...90018428911617
This is not true. My guild had an offnight due to the Halloween holiday and decided to clear the last two bosses in Mythic NH to get someone a mount with what we had on. Because we've had a lot of turnover, we have had to reteach this fight and deal with wipes. We didn't have enough people online to do KJ progression, so we just used the 19 people in the raid to kill Guldan instead.
https://www.warcraftlogs.com/reports/zcfNm6RAgPLbTZJq
Upon killing Guldan, we found there was no mount. We found the tweet above, and I contacted customer service and filled out a ticket about a loot drop issue. Here is the communication that occurred with GMs.
GM reply to initial ticket
My response
2nd GM response
The GMs were nice in explaining that the tweets and information from Blizzard were not quite correct.
Bottom line: despite the comments on wowhead, this is not a 100% drop chance unless you have 20 people. You will need to fill the raid to have a reliable mount drop.