Before I post; To flamers and trolls, I searched "Stay mounted", "Stay mounted in instance", "mounted in instances", "Mount exploit" and nothing came up, as well as nothing comming up to the title I entered, so I apolagize if it is a repost but I did not find it anywhere.
Hey guys! Just posting something cool I found as I was doing my random today. I was playing a 76 hunter and I did my random WoTLK dungeon for my 2 emblems and when I accepted the summon I was mounted. When I joined the instance I stayed mounted and could move and I kept my speed increase! (I was in Zul'Drak). I asked a friend of mine, a 73 DK, to try this and he gladly accepted and got the same results. Not sure if it matters but I am playing on Cenarion Circle (US), just so you all know.
Short version:
1. Queue for instance
2. Be mounted when the "Accept/Decline" option comes up
3. Accept the invite and join the instance, you will stay mounted and get speed bonus.
4. ???
5. Profit
Hope this helps some peeps, enjoy!
Edit: Tested in a few more and I could stay mounted in Violet Hold and the Old Kingdom as well.